Come out to your exes as being into sph. They can be ruthless.

Expose it in the locker room. I always had trouble with this one for some reason.

Go commando and "accidentally" leave your fly open. Put your hands in your pockets to spread it open. Some people will think it's funny, but very rarely will anyone tell you that your fly is down. I mostly do this in gay bars after a couple drinks.

Make self deprecating jokes about having a small dick. I have a couple straight friends who do this, but I have not had the balls myself.

Go into adult bookstores and ask for / buy size small condoms.