I would say the best way to do this is to make sure that the person or people that role play with you understand that you are in the role play for the long term goal and set an agreeable start and end point. Take that extra time to make sure you have all the fine details hammered out in your rp. Maybe even do a series of small rps with your partner building characters that you would like to use in future role plays that way when you go to use them again they already have a rich history and back ground like many of the more popular comic characters. I have been playing with the same person on a different site for just over two years on the same role play because we were looking for something short term and by the time we had realized it was no longer short term we didn't want to stop. Now we play nearly ten characters a piece at any given time. Well thats my advice, hope it helps in some way.