All you need is a rack and panniers (bike bags) attached to the bike. Two separate panniers are much better than just a big one (it is easier to take them off, and we need to do it every day). There’s also the possibility to carry four – two at the back and two at the front.
Bike Panniers which are separate, light weight, waterproof and a good shape, can be pretty expensive. On the web you can find several DIY projects on how to make your own from (recycled) material (e.g.: here).
Some people take a trailer instead of a rack and panniers, this is to your personal choice. However, first of all you will never get a day off from riding with a trailer (which makes riding different in sharp turns, downhill etc.) and secondly it prevents you from carrying the communal trailers, a task which we would all like to share. If you do so maybe you could think of volunteering to carry some food or material on your rack instead sometimes.
The second thing is: don’t take too much personal stuff. Try to put everything into panniers and check the weight: they shouldn’t weigh over 15kg for the 2 panniers together.
Last but not least: maintain your bike regularly. You will notice quite a big difference after properly inflating your tires and oiling your chain. On every bike tire, there is a pressure recommendation written. Even for cheap tires, the maximum recommendation is usually between 3,5 and 8 bars (depending how thick the tires are), which is far more than most people would assume is safe to put in. Inflating your tires to the maximum pressure will not only make the luggage be pulled much more easily, it will also make your tires last much longer under weight.