Well I find that I can get 'into' the story again if I read back through the past few posts when I'm forum posting. Having said that, I have found that sometimes it clicks and sometimes it doesn't. Also, it depends at least in part, I think, on whether the participants have compatible ideas about what they want in a story (e.g., does it build up slowly to the action, or is it rapid-fire back and forth type stuff?) Overall, I have found that forum posting works pretty well for me.
I don't like IM roleplaying, particularly not of an adult nature. The major reason why is I have met one too many weirdos (and I don't mean the good kind of weirdos) who don't believe me when I say that I want RP, not just cybersex. Also, I think that the rapid rate of IMing makes it harder for me to be as descriptive. For me, sometimes I read a forum post, think about it for awhile, and then come back to post my response, which gives me time to add more descriptive elements. And, I find forums work better for rules-based games, which I enjoy.
Just my 2.5 cents.