Keep drinking water and you'll have to reach capacity eventually. Unfortunately apart from that and being in an inconvenient situation without a toilet near by I don't think there's anything you can to to make yourself piss.
Stop, close your eyes, lean your head back, breath slowly but deeply, and let your mind go blank.
Works for me every time, although since I'm tall enough to see over the panels in most public bathrooms it makes for sometimes awkward situations and undesired accusations.
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Alternatively, turn on a faucet.
Works for me every time, although since I'm tall enough to see over the panels in most public bathrooms it makes for sometimes awkward situations and undesired accusations.