SITC, that is an excellent reply. I too find it difficult. I had my own self employed tutoring business and found it hard asking for money at first - and these were people I didn't know - but whast was harder was when parents wanted to know how their child was doing so we'd talk for a few minutes, then they would turn to go and I had to say, "Er...you haven't paid me." (the 'er' makes it sound less confrontational I think). It was always an oversight and they would apologise. Must admit, it got easier as time went on.
As far as your friends are concerned, have you tried saying, "You haven't forgotten you've not paid me yet for...?" If it is an oversight, this should do the trick. If that has no effect, I would give them an invoice. Any objection and you point out that you doing it as a business. I like Yorkshire Lass' suggestion for dealing with the craft swap.
Good luck.