It's very easy to spend money without thinking about it- contactless payments and paying for things with paypal online are my biggest issues.
Things that can help:
-Check your bank account regularly.
-Set a budget for socialising etc for the week, draw the money out in cash and don't be tempted to spend more.
-Don't take your card out if you are drinking.
-Make a weekly/monthly budget and stick to it. Make sure you know what you're spending on each bill and your weekly shop etc.
-If you're earning money, try to put a small amount into a separate account each month. Think of this as an emergency fund, and don't touch it unless you really need to.
-Try to avoid spending money on small things online- yes, ebooks (for example) may often cost less than £5 each, but if you buy £5 and don't read them, that's £20 you've wasted.
-Check you don't have subscriptions to things you aren't using.
If you're really struggling to work out where your money is going, a spending diary can help. Write down everything you spend for a week/month and you'll soon realise where it's disappearing too.