Ok, I've been cycling the 12km to work every day for the last 3 months now and I thought I would share some of the things I have learnt over the time.
1. Get a decent bike and look after it. If you're doing any sort of decent distance, a road bike is best, but if you have to use a mountain bike, I can't recommend slick tyres enough. They won't cost you much and it will make life a lot easier for you. With your tyres - make sure you pump them up to the maximum recommended pressure and keep them that way (the rubber leaches air over time). The pressures may seem a bit high (120psi on mine), and you might need to get yourself a good foot or floor pump to achieve this - but don't worry they will easily take it. Doing this will give you a better ride, your tyres will last longer and you'll avoid pinch flats. I've had 2 punctures so far - and both have been from when I hadn't pumped the tyres up enough.
2. Buy a rack and panyer bags. Rucksacks make your back sweaty and disrupt your balance if you have a lot of stuff in them. Riding without carrying anything on you is a much more pleasant experience. If you can't afford the full panyer bags - I just got a bag for the top of the rack which has little flap down parts for the sides if I need to put more stuff in. This is quite adequate for going to work every day.
3. Get clip-less pedals or at the very least toe clips. I thought this was a little too much when I first got my new bike, but once you've tried the clipless pedals and proper shoes there is no turning back. You'll get at least 2 times more efficiency from your pedaling as you are able to apply pressure through the full 360 degrees - it cut my journey time by 5 minutes and I am a lot less tired. Another good tip is to use a lower gear and increase your cadence - it may not seem like it, but you get better efficiency.
