A clean, degreased gear system will outlast and outperform one that has not been maintained.
However! It must be done properly. If not, you'll actually end up accelerating the wear on your components.
We recommend a chain bath: the brushes work wonders. Don't have a chain bath? No problem: take your chain off and stick it in a can or bottle (or container you never want to drink out of again) along with some solvent such as kerosene, turpentine, or degreaser.
Now shake it like a Polaroid picture. (Ah, dated musical references FTW.)
Keep refreshing the solvent until it remains fairly clean after agitation.
This is critical.
If you don't get the solvent out of the chain, it'll stay inside the rollers. When you then add lube, it'll turn into a cutting oil and chew your chain to shreds.
Worn chains lead to worn cogs which in turn leads to cranky cyclists who think Shimano are purposely making parts that wear out too quickly. But we digress.
Get some very hot, very soapy water. Add it to the can/bottle/container your chain is in and shake it about.
If the chain is still on the bike, smother the chain in dishwashing liquid and put clean water in your chain bath, then pedal backwards to really work it through. (See that black crap rising to the surface of the water or coming to the outside of the chain? That's demons...er, solvent and residual contaminants.) Repeat this process until there is no more blackness coming out from the chain.
Rinse off the soap using boiling water either in the container (shake it!) or by repeating the pour-and-pedal routine until the chain is clean and shiny.
Leave it in the sun to dry off, then lube.
To clean the chain rings, cassette and jockey wheels, it's best to remove them from the bike. (If you don't have the appropriate tools, just take the rear wheel off and tip it so the cassette is angled towards the ground and the solvent won't run into the bearings.)
Get a stiff-bristled brush and your solvent, then scrub away at the cassette.
Do the same for the chain rings (again angling them away from the crank bearings).
Now wash with hot, soapy water, then rinse with boiling clean water.
You're done! Now, wasn't that easy?
If this whole thing sounds like a complete palaver, just over-lube the chain, let it soak overnight, then buff off the excess.
Although it isn't perfect, it's much better than degreasing improperly or doing nothing at all.
While degreasing your drive train, care is needed to not splash solvent, soap, or chain lube onto either your disc rotor or -- if you have rim brakes -- the rim’s braking surface. Contamination of this sort will be absorbed by the brake pads, leading to a very high chance of having almost no braking power on your next ride. Yikes!
Inspect your rotor/rim carefully after you’ve finished working on the drive train and, if you’re at all in doubt, wash the surfaces down with alcohol and a clean rag.