DIY on an oil change I did on my 1996 BMW 328I, takes under 30 minutes to complete, saves you money, and you can be assured that your doing it right.
I like to do my oil changes right after I have driven the car, this is because the hotter the dirty oil is the better it will come out and remove the gunk that has built up in the system. 
Always use synthetic oil when changing your BMW's oil, and try not to let the bolt fall in the oil pan or you'll be sifting through hot oil to obtain it.