lol same here, its a nightmare, within 10 seconds hes on his front. I was on the floor the other morning, and saw the funny side, It'd taken me about 6 mins and I was no-where near getting it on. Ended up laughing and letting him crawl away naked lol.
Have developed a skill of putting it on him while he's in crawling position! took a bit of practise but dont have to pin him down now!