If your friends come to your home
The absolute key to DIY is to get someone else to do it. This might seem counter-intuitive but once you see the results, you'll be amazed by the effectiveness of this approach.
Simply hire a carpenter or painter to come to your home before-hand and provide him with the equipment and materials he needs. This gives him reason to leave them at your home as well as any left overs. When he has finished the work, simply leave your home in a mild state of untidiness, good tips for this are sprinkling sawdust or dripping paint. Make sure to get some on your clothes.
For an added bonus, leave out the receipts for your DIY purchases in a conspicuous place by the finished products.
Other methods include commissioning the creation of trophies from any good trophy maker for awards such as 'Surrey County 2004 DIY Competition Award, 1st place' or other such mementos of your previous acts of DIY prowess.
If your friends wish you to perform some DIY for them
There are two approaches to this which I will reiterate for you now.
The first approach is to simply follow the same rules as if they were to come to your own home. Tell them you work best alone, that you need space and that they should leave the house or go on holiday. When they return to a beautifully finished product you can claim all the credit.
The second approach is to simply take a supervisory position and insist that it's called DIY for a reason. Offer your help and good advice but engage only in the most minor of actions or restrict yourself purely to handing tools to your friends and making them tea. If the results are good, it was all thanks to your supervision, if not, your friends just couldn't follow your instructions properly, quite obviously not being as gifted at DIY as you are.
~ problems solved ~