Colon cleansing questions

I'm IBS-C predominantly, but even when I am going once a day, I suffer from horrible bowel spasms. I've had constipation problems for years and I'm considering having my colon cleansed and have found some places in my city that do cleansings (I don't want to try one myself - too scared!). My questions are: what is the best kind of "cleansing" to get done? Is one session enough (some of you mentioned that you've gone a couple of times a week, which I definitely could not afford!)? Is there any preparation you have to do before you go or anything you have to do/avoid doing after its done? And lastly, a stupid question - if the cleansing makes everything come out, will everything keep coming out afterwards (i.e. will you get diarrhea or will your body readjust itself)? I'm worried that the cleansing will worsen the spasming and that I'll end up constantly feeling like I have to go. Thanks for any help and advice!