urinate / pee / potty

URINATE / pee (general)
This is the sign I recommend parents use with their children (both boys and girls).
Touch the letter "P" to your nose while puffing out your cheeks a bit.
Note: it is the tip of the middle finger that touches the tip of the nose.
Note:  I used to use this sign a lot with my kids when they were real little.
I'd look at them with raised eyebrows and sign "NEED PEE?" to ask them if they needed to "go potty?"
Note: If you don't puff out this sign, it can be used to mean "p_nis." (The male bodypart). 
It is very much context based.  If someone signs, MY "P-hand to the nose" HURTS -- well then you can assume that either his organ is hurting him or perhaps the process of peeing is painful.  If that person signs I NEED "P-hand to the nose" -- you can generally assume they mean that they need to urinate.
This is the sign I used with my kids as they were growing up to ask if they needed to pee.

URINATE / pee:
Okay, so, this version is specifically for "guys." I don't use it with my kids or everyday conversation.  I'd only use it if I needed to specifically describe the act of a male taking a wiz.  I thought about not including it here, but since many interpreters, social workers, and or group home workers need to know signs like this I figured I'd include it here.
Take an index finger and hold it a bit upright then point it down and forward as you puff out your cheeks.
I do not consider this sign to be very polite nor very useful.