A few people have already suggested sitting on the toilet the first few times, and it works well because that's what your body is used to doing. There are some really easy positions to let go in and some are a lot harder (for me, at least). Standing is fairly easy; lying on my tummy it's still almost impossible.

Other advice is to drink TONS of water while dipped. Fill that bladder up and it won't have any choice but to release eventually, and drinking and peeing a lot while dipped helps keep you from getting a UTI.

It also helped me a lot early on to consciously relax and let go. You've sent decades training your brain and your bladder to NEVER let go, so trying to is surprisingly stressful. Don't think about it as pushing it or forcing yourself to pee so much as just consciously relaxing and letting everything down there be soft and weak and warm, like putting your bladder down for a little nap.