I think diuretics aren't actually good in cases like this as they can put too much strain on the kidneys with the additional pressure of urine trying to get out. Happy to be corrected though.
I had several large kidney stones and was thus taking a lot of opiates for the pain and suffered from urinary retention. I found three good ways.
1) Fill up a sink (plugged) with luke-warm water, did your chap in and just literally let yourself melt - let go of all of your tension, your anxiety about not peeing, the pain, the discomfort... just let go of every muscle you're contracting. Usually the luke-warm temperature will induce urination. If that doesn't work you can leave the warm tap running in the sink and splash about a bit... I guess it's that psychological water splashing thing that makes you want to urinate.
2) Have a warm shower and pretty much do the same as above...
3) Sit down on the toilet, as opposed to standing up, and just completely relax. I find that when you're stood up, you're a little more tense, but when sat down and your body muscles aren't tight and have relaxed, you've got a better chance of urinating.
I guess everyone has their own little ways... Unfortunately I had to be catheterized in the hospital because of severe urinary retention, but that was more to do with the kidney stones than the opiates. Oh, I was also put on a beta-blocker called FLomax/Tamsulosin, that really helped after the catheter was taking out. Could pee like a baby. It's available OTC in the UK.
Just be patient, set a long time aside and don't feel rushed. It WILL come.