I've always practiced sitting down and now I want to stand. So I have a Les Paul and a really nice PRS Leather strap. But the neck keeps tilting down. This is driving me crazy. I thought maybe it was the shirt I was wearing was too slippery but that isn't it. I feel like I'm holding the guitar into position all the time which is hindering my playing. I'm not a gunslinger, I want it only about 30 degrees from vertical. But I'm pulling up on the neck constantly. What can I do ?
A lot of players found that where the guitar is when seated can be very different from where it hangs when standing. A simple solution is to change the strap length so that the guitar is pretty well in the same place for both - usually higher when standing.
This has lots of advantages, mainly that angles don't change between sitting and standing, so you won't need to relearn some of the ways you play.
If it's an old Les paul, it'll have a heavy body, so head droop usually won't be a problem. Newer models aren't so heavy, but still it's not usually a problem. The further up your body the guitar is, the less of a problem this should be. 30 degrees from 12 o'clock seems an odd angle to play at, more horizontal is more favourite.
Tighten the strap. (Assuming both ends of the strap on the guitar? ... vs. the guitar and headstock?)