I agree with the others – stay hydrated and maybe drink coffee to give yourself a boost. But I have another suggestion which works for me but not for my husband so I have no idea if it’s unusual.
As long as I haven’t just very recently peed, I can make myself pee by tickling the top of my ass crack. So I sit on the toilet, touch that spot, and pee starts to trickle. Then it’s like as soon as it gets started, it’s easy to continue.