Sitting down is great advice or often I would put a hand against the wall and lean, relaxation seems to help a lot which segues into my next tip - don't force it. A lot of people think that 'pushing' harder will help but when it comes to opioid-induced urinary retention (the technical term for this phenomenon), it actually makes things worse.
I always found that relaxing as much as I could physically and mentally, almost meditating, made it a lot easier. If you have to piss really bad and you're desperately trying to push it out, it just won't work and will frustrate the fuck out of you. You have to relax and let it come. It took years for me to master this and even after I figured out that you can't 'force' it out initially, once I got a stream, I would still try to get it out faster by 'pushing' since I had to go so bad. The stream is going to be weak and when you have to go really bad, this will be quite annoying, but you have to stay relaxed and just let it trickle out slowly. If you try to push, you'll just stop and frustratingly have to wait again to get it started.
With a little practice it becomes easier but between this, tearing your asshole apart every few days (hopefully), spending all your money (or having to do fucked up shit just to acquire some to get it) and the enjoyment of it decreasing over time until you are just trying to not be sick as fuck, how long until it's just not worth it? I don't want to preach so I won't go further, but when it gets to the point that you CAN'T piss at all and are starting threads looking for help, maybe it's time to start asking how bad you're willing to let it get.