Regularly cleaning and lubing your bike's chain reduces wear and improves shifting performance. Worn chains are bad karma. They drastically increase front and rear cog wear. Replacement drivetrains are expensive, so looking after the business end of your bike is well worth the effort.
Remove the crusty bits from your chain, derailleur, rear cluster and front chain ring(s) with a BBB Chain Brush. A tooth brush works ok too, but beware the wrath of your flatmates if they find out. A small flat blade screwdriver can help with stubborn detritus.
A BBB Chain Cleaner with various rotating brushes will clean your chain to within an inch of its life.
Top up the chain cleaner with the degreaser and clamp to your chain. Gently spin the pedals backwards - the brushes will whizz and whir to clean the chain. It may take a couple of goes with a fresh solution each time to get it spotless. After 15 minutes attack with a clean rag to remove any recalcitrant gunk - the degreaser should have worked it's way in to soften it up. Leave for a while to dry.
Then apply some Biomaxa Chain Lube. It's an effective, long-lasting lubricant for both summer and winter conditions - plus is easy on the environment. Squirt onto the chain while spinning the pedals slowly backwards. Fanatics lovingly squeeze one drop at a time onto each link - your choice. Once applied continue to spin the pedals for a couple of minutes to help distrubute the lube and coat the chain pins. It's best applied the night before you ride to give it time to make like Colgate and really 'get in'.
Always apply lube in warm conditions - squirting onto a cold chain causes it to form nasty solid clumps. A hairdryer to pre-warm the chain is a good plan. Again - be stealth-like in borrowing and returning to your flatmate's room.
Wipe off any excess lube with a clean rag. Be sure to never let lube get anywear near your disc brakes.