Pair Up When You Can

One of the tricks I've found over the years to help get, and keep, a PC in the game is to partner my character up with another player. Even if the game is starting with the group together, having a longer living relationship with another PC gives something to fall back on. The best part is it is almost harmless to do. Do it right, and you have something to fall back on and help enrich the game, your RP, and everything else. Do it wrong, and you're basically left with a normal PC in a normal game.

Life Before The Group
The key to this is having a life before the group. Spend some time and talk about what your PC and the other PC (or PCs if you do a group of 3) did before the game started or the group got together. How did you meet? What did you do? Why did you stick together?

This takes a bit of planning. You want to make people who get along, but you also want to stay true to your character concepts. The idea is ot have something that helps expand your character and the game, not something that prevents you from playing what you want.

Define Some Awkward Moments
Have some awkward moments in the game. One for each person. Maybe it is how you met. Maybe it is some other, previous adventure. But have something where one PC looks foolish and the other got to be the savior so to speak. This will help endear the relationship and cement it for both of you. Odds are you have no idea what it feels like to face off against a master swordsman in a duel on a snowy plain like your character does. However, you probably know what it's like to be embarassed in front of a crowd, and how it felt when your friend saved you - or how much you wished a friend did save you.

Try To Talk About Personalities
Like I said, personalities that mesh is going to be key, so try to have it defined as best yo can before hand. If you have exercises you do that help you envision the character and how they act, go through it. Make sure the PCs would get along. If not, explain how it works that they like each other and are still together. It's fine for opposites to attract, just know that is what you're doing going in.

Revise and Revisit After Play
No plan survives first contact, and that is true for prior relationships. This is one of the reasons I tend to not start with pre-defined romantic relationships for characters, unless it is with a player I know can play into it in a way I expect. Too often you get into game and find out that the other PC is not who you expected - or your PC is playing differently - and now your pre-existing relationship feels flat and can't come into play. You end up with one character hanging out with other PCs and the supposed best friend abandoned and alone.

This is ok. It happens sometimes. Like I said, no plan survives first contact. However, when you notice that it happens you want to address it fast. The earlier you talk about things with your partner, the earlier you can fix it or agree to abandon the plan. So keep an eye out, and be prepared to revise things as needed.