You need to have self restraint and control to keep it no'strings. It is not for everybody.
Some things that keep it no-strings:
Don't be interested in them as a person. Don't follow their every move on facebook or give a shit where they go at the weekends. You cannot start 'caring' about them.
Don't keep any one arrangement going too long, incase one of you starts getting 'attached' to the other one.
Don't stay the night or have them stay the night at yours.
hmmm, what else? I guess just don't think about them, apart from when you are thinking of sex. Don't even entertain the thought that it might be more.
Keep contact to a minimum. Simple requests to meet up, simple yes or nos in responses to their requests (I'm thinkin texts here)
Have had a few arrangements from various websites, and a few times with people I met through friends.
Once with a friend but miraculously it worked and there where no tears and no loss of friendship afterwRDS.