I went to London in June 2008. Here are some things that helped me-
1. I purchased a great book on amazon.com for $11.04 called "Frommer's Best Day Trips from London: 25 Great Escapes by Train, Bus or Car". This book was great! It listed sites and activities to see in places that can be reached and enjoyed for a day such as Bath, Oxford, Canterbury, Stratford Upon Avon, etc. In fact, it was so detailed that it listed a wonderful guided walking in tour in Bath that I would not have known about otherwise called Jane Austen's Bath. It focused on her residences and settings for her novels. It's amazing how many places in Bath are still around that she used. The BEST thing about this book is it told you which London stations to go to for specific destinations, which train to take, when they left, how many ran a day, and how long the voyage would take. I never has to plan or map a London to X destination.
2. Get a Great British Heritage Pass. It will get you into many sites for the price of the card. If you see a lot, you can sae a lot. They don't list all their places that are covered, but you can contact them and they will tell you how to get a complete list.
3. If you will be taking a train, get a Britail Pass. You can get on and off trains as much as you like for the rate you paid before you left-a one time fee for so many days. This is good for those day trips out of London.
4. If you are departing from America, take British Airways. I took Delta a while back and it wasn't as nice for almost the same price. There was free alcohol, socks, eye patches, pillows, great bathroom access, and friendly staff.
5. Book your hotel a day in advance so that you will have a room ready when you arrive at your hotel. I have taken a trip to London twice with London Week with Trafalgar Tours and the room was always ready later in the day. You don't have the energy to site see due to jet lag. Having the room ready, means a more rested first day and being able to go out later in the day rested.
6. If you are going to be in London briefly, my must sees are Westminster Abbey because monarchs and famous people are buried there, Tower of London, and the British Museum. The British Museum has very famous artifacts in it. Big Ben and Parliament are a hop and a skip from Westminster Abbey.
7. Go to your local bank and get some pounds before you leave so you will have some money on hand before arrival. I took 100 pounds in paper money.
8. VERY IMPORTANT-Get travel insurance before you book anything. I used TravelEx. They cover you in case you get sick abroad. Also, if you can't go before the trip due to illness, a relative dies or gets sick, or something happens to your traveling companion, you are covered. Look at the plans and see what you want and what coverage you want. They say to never get insurance with the comppany who books you.
9. You can get MedJet Assist for overseas emergencies. If you were hit by a car, had a heart attack, had a fall and had to be in the hospital, this company will fly you to any hospital in the world of your choice. Check for fees and plans. It was pretty cheap when I got it. It would get you home when you need to go home.
10. Make sure you get an adapter for your hair dryers, etc. They don't use the same outlet types as the U.S.
11. Let your credit card companies and bank know you are going abroad so they will not stop charges for suspicious activities.
12. When choosing a hotel, ask if you can control the climate in your room. Some have limited control in air conditioned rooms and it can be horrible. Make sure you have your own ability to make your room cold and not slightly different. I learned this in a brand new hotel the hard way.
13. They didn't have free refills in most restaurants on sodas so ask for a glass of ice and then the soft drink in a separate glass. You'' get more.
14. Take a digital camera! You can see how well your photos turned out and if you need retakes. I got my camera for $100.00 and it was worth it! You also don't have to worry about the film turning out.
15. See a play! They have great theatre and many times with famous names! Look on British theatre sites for what's playing. You can do this in the evening after site seeing.
16. Try looking at packages for discounts and goodies like those with companies like Trafalgar Tours. This way, all the big stuff has been paid for before you arrive and you don't have to see to reservations and such.