It's easy to get lazy about leaving your postcode, but do it; you won't regret it. Go beyond the local pub every now and then, because there are plenty of other neat places just around the corner.
London has lots of student/youth discounts, so always ask or check. These should be a thing everywhere. I'm looking at you, America.
Europe is cheaper than London. (Everything is cheaper than London.)
Booking tickets and planning trips can take a lot of time, but it's worth it. You'll know what's there to see and do - and how to do it cost-effectively - so you won't return and realize you've missed out on something wonderful.
If you're a tourist visiting London, go to the countryside. I say this hoping to reduce crowding, but it's also quality advice. Hop on any train and see how much there is in England (or Scotland, or Wales) that you won't have to squeeze or queue for.
If you like exploring, gardens, beauty, and history, Open Garden Squares is the best £10 you'll spend in this city.
Tipping is nice, but not expected. I wish America would learn this/pay a proper wage; I've been very confused since I got back.
London is having a bit of a thirties-forties moment. Which is slightly odd, considering they never had the need for speakeasies (no Prohibition), and considering the general trauma of the period. Eh, history gets recycled all the time.
"Quite" does not mean quite what you think it means.
Knowing how and when to make a good cup of tea - using fine bone china, of course - are vital life skills.
It takes about 9 months to settle in and really feel at home somewhere. If only one could stay...