In response to up all nights questions, i'd say that approaching a girl and telling her she's hot is a stupid way to do it. a) because she's prolly heard this a million times and b) because it will put her on the defensive.
The best pick-up line is "hi"
My advice would be to find an 'angle' into conversation. Say you spot someone u want to talk to in a shop, looking at clothing, comment on the clothing. Or at a bus stop, you could ask them where they're headed. Basically, strike up a conversation based on your surroundings so its not such an obvious 'i'm trying to pick you up' - that is bound to be met with natural resistance even if they are attracted.
If you do compliment, do it sparingly. It is awkward to be showered with compliments, especially by someone you don't know. Compliment then continue in the flow of the conversation. Don't make a big deal of it.
Breakabeat - those relationships are possible but i guess both people have gotta be in the same mindset. Fun lighthearted, casual relationships should start on that basis and both parties should be aware of what the nature of the relationship will be. If both are cool with it then thats perfect. If one person is looking for something different then its not gonna work out - its best to clear these things up straight away to avoid confusion and emotional engtanglement.
To what DJC said about friends being a basis for a relationship - That is true, theres always that risk of losing the friendship which is why its probably better to try and make your move BEFORE you become friends. If it doesn't work out, you can still be friends after - knowing that you at least gave it a shot. It will save alot of emotional anguish in the long run.
Adikkal