Keep to yourself. London is an enormous city whose residents are largely on guard in public. Like any major city, being overly friendly to strangers in public is a sign that you're a tourist and not accustomed to being in the city. Refrain from unnecessarily talking to strangers on the Tube. Bring something to read to stay occupied.
Use the token machines unless you have some reason for not doing so. If you want to look like a native, avoid standing in line to buy your tokens. Go directly to the machines and buy them like you would buy something from a vending machine.
Study your map before you get to the Tube. If you enter the Tube, ask for a map and then stand to the side and study it for 20 minutes, you are an obvious tourist. A local either already knows the route or scans it quickly from the maps affixed to the walls of the station.
Pay attention to what time of day you are traveling. If you are taking the Tube during rush hour, dressing in business attire or in business casual clothing will help you blend in better. If you're out on a Friday or Saturday night, far more casual clothing will be expected.
Hold your purse or shoulder bag close while riding the Tube. For women, wearing the shoulder strap across the body to make it harder to snatch is not unusual. For men, wearing a backpack on the front rather than the back is a normal way to wear it in the Tube to avoid pickpockets.
