The best plan is to always make a list. Decide how many days you're going for and work for that. If you're on a weekend city break then you'll need far less clothes than if you were on a big girly holiday and plan to be out every night in bars and clubs. Play it by what you know. Don't overpack though. Only take as many tops as you will actually need. (Don't take 3 more 'just incase'..) Obviously the list is only a rough idea of what things/how many of them you'll need, but its good to keep it in mind!
Don't take a huge suitcase if you dont need it. The bigger the case, the more things you'l be tempted to take! (Remember your luggage allowance!)
Leave space for the things you will (undoubtedly) bring home. Bottles of cheap alcohol, presents, new clothes...
Leave the bits you really don't need to bring home with you there: almost empty bottles of shampoo, shower gel, toothpaste etc..
Invest in a strap to go around your suitcase. Or even put some stickers, ribbon or a distinctive mark on it. I cant tell you how many black/red/green/blue suitcases that go around the carousel in the airports.
Also, get a padlock that goes through the zips. It doesn't seem like much, but anyone thinking about taking yours may think again if they see it as it's too much effort! not only that, airport baggage handlers wont see your pants because it has no chance of opening!
Don't keep all of your money in your hand luggage. Its always a good idea to keep some in your suitcase too. Just incase you forget your bag somewhere or it gets stolen. People in and around airports aren't always there for the same reasons as you!
If you get holiday insurance, make sure your suitcase and hand luggage is covered too. (just incase it gets lost! or ends up being on a later flight!)
-Do you make lists and stick to them? what are your packing tips? let me know! x