if you go out shopping with ur friends make sure ti bring only just enough for food and thats it (so ur friends cant tempt u to buy some other stuff and they cant "borrow" off u either XP)
u can also get this app on ur phone that helps you to budget and plan ur spending for u making sure u dont go over the limit.
or u can simply ask ur mom to keep some money for u until u seriously need it.
hope this helps :)

Chant a silent mantra while eyeing lots of K-Merchandise. HaHa.:)) This has always been a problem to me! So Self-Control is the only thing I can advice. ^^

Hmm restrict yourself to a budget every week or month or something.

Open a bank account...