Bonding an RPG party.

One-Shots & Short Campaigns

If you're running any type of shorter campaign, party bonding will be trickier. People are generally less invested in their characters as they know the story will be over soon, there's less time to form actual bonds, and less time to make sure there are bonds through some methods, but that doesn't mean shorter campaigns are doomed to be less fun or less memorable than longer campaigns.

Pre-Defined Relationships

One easy and effective method is to make sure relationships are defined at character creation. It's important to be fairly rigid here, characters will need to have a bond strong enough to be willing to work together in the manner the campaign will ask of them. Loving siblings will likely be willing to risk their lives for each other, for example, but ex-lovers who ended their relationship violently might not be as willing to do so.

These pre-defined relationships can be created both by the players and the game master (GM), but the latter works better if the GM fully creates the characters, rather than just their relationships. Involving the players in this step, if possible, can also help with bonding the party as the players will be able to play off of each other's ideas.

Railroaded Story

Some groups prefer more freedom when it comes to creating their characters, and in this case it can be more beneficial to have the story be more railroaded. The characters won't need to know each other beforehand, but they'll need to be forced to work and/or interact together in some way. Perhaps they need to escape an assault on the ship they're all traveling on, or perhaps they were all summoned to a court for a reason yet to be revealed to them.

Putting them all in the same difficulties is an effective way to form bonds, but isn't strictly necessary. The characters could be scattered around town, all doing their own thing, but then have to face a threat to the entire village. Here it can still help to have some pre-defined relationships to the town or pre-defined character traits. A character without a bond to the town or without a willingness to fight your threat might simply flee town or hide in a basement somewhere, which is usually not what we want.

Longer Campaigns

With longer campaigns it all gets a little easier, but at the same time potentially more difficult. There's more time to form bonds, more freedom to have characters not follow to plot if it fits their character traits, and more opportunities to force bonds to come about. At the same time there's also more time for characters to stay separated, giving the GM more work.

Pre-Defined Relationships

The pre-defined relationships concept works in longer running campaigns as well, but has the added bonus of offering opportunities to play with this background story a lot more. Those two siblings I mentioned earlier, they offer a great opportunity for a kidnap and hostage situation later down the story line. A family member will be more willing to pay a hefty sum than a new friend after all. Those explosive ex-lovers, they might be forced to work together, one-on-one in a stressful situation.

The downside of pre-defined relationships is that they might not feel as special as those developed during the story, but both can happen within the same campaign. Characters might not have a relationship with every party member, for example, so both types of relationships will occur.

Forced to Meet

Both the players and the GM know the game is generally about a party overcoming obstacles, so a meet up is generally inevitable. It can be as simple as all party members requiring the same mode of transport or using the same tavern, or as complex as you want it to be, but it generally doesn't feel forced when the circumstances are natural enough. This method is very effective, and there's a lot you can play around with too. Perhaps one of the biggest elements you can use to your advantage is setting things up in this meeting of characters. Whether it's setting up the area, an event or a single NPC, there's a lot to play around with.

For example, the meet up could happen at a broken bridge (location), at a wedding of people all the characters know (event) or in an unknown location a kidnapper took all the characters to (NPC). They can all be combined as well of course, the first part of the campaign could open at a funeral of a deceased acquaintance of all the party members. A specific, plot important person may be speaking about the deceased, and the location of the funeral may be quite eccentric to reflect the wishes of the deceased, which in turn might be why the party is attending this funeral, for example.

Initial Danger

It can be tempting to start the campaign of with a mighty adventure of great peril and reward, you want to hook your players immediately after all. However, this won't always work well with every party, as a party of strangers is only willing to do so much for each other. While a good reward might entice some characters to work with strangers, trust and risk are hard to balance against reward. Obviously with some groups the players will meta-game a little to go along with the flow, and this is not bad at all, but some players prefer to stick to the character choices they've made during character creation.

Regardless of whether players should or shouldn't bend a little to go with the plot, it makes sense from a realism point of view to start of with smaller challenges too. It's something to keep in mind, but it's ultimately up to what you and your group want.

Meet Up Later

If you, as the GM, are up for the task, you could always allow your players to meet up at a different point in the campaign. This does mean you have a lot more work cut out for you, as you'll have to track twice or more times the amount of locations and NPCs, but it could be well worth it. This is hugely dependent on your group though, as not everybody enjoys being a passive observer of a game they're part of for half their game time.

Player/Character Types

Sometimes you might simply need to discourage or ban a certain play style or character type. An evil character in a group of good characters generally won't work well for group cohesiveness, and edgy, lone wolf characters who refuse to group up with people because it's against their lifestyle aren't exactly great for gameplay either.
Ideally you also want characters with similar enough motivations. While you can definitely have somebody who just loves money and somebody who loves to do good in a party, things get a little trickier with more extreme characters as far as differences go. Not impossible in some cases, but not always fun either.

Alternatives

There are some alternatives you can play around with, but these aren't for everyone. They can still be worth keeping in mind though, even if it's just to alter them a little or mix them in with other ideas.

Multiple Characters

One way to get around the issue of party cohesion slightly is by allowing the players to create multiple characters, but only play the one who fits the quest or struggle at that specific time in the game. This way there might not be a set group, but merely a bunch of people who travel the world and work together once in a while. Depending on how many characters you have, it could make the world seem more lively as there will be more heroes around, rather than just one group of adventurers. The obvious downside of this is all the extra tracking required by everybody. Still, it can make for a fun mechanic, as well as for a way to play different characters that may not always work. Even the edgy lone wolf could come in once in a while.

Fluid Characters

Another, perhaps more meta gaming solution is to have characters be less defined at the start. When a character choice comes up, the choice is made in favor of the plot for as long as the choice hasn't been made before. This can be a fun way to discover a character as you play, but it can also be difficult to keep track of the character choices already made.

Collective Story Telling

Instead of trying to create pre-defined backgrounds or going through a smaller adventure, you could also have a session in which you collectively tell a story of how the group overcame an obstacle together. How everybody met, who became friends or who had a little more friction, and any other detail you may wish to add. The great thing about this is that it becomes a game in and of itself (you could even use a game system specifically designed for this kind of game play), it allows players to play off of each other's ideas, and it helps set up a great backstory for the characters, and for perhaps a vast amount of NPCs, locations, and a setting history.