Not only does it look nasty and make cycling more difficult, all that muck and grime can also wear and rust your valuable components.
Luckily, keeping your bike clean and in good working order is dead easy with the right technique and equipment. Just don't forget your toothbrush.
Bucket, hose (optional), sponges/cloths/brushes, toothbrush, dry rags, bike cleaner, degreaser, chain lube, protector spray
First you need to get the worst mud and clag off, especially if you're a mucky mountain biker. Using a hose or a bucket of water and brushes, remove the worst of the grime. If you use a hose, keep the pressure low as a powerful blast of water could wash grease from seals and bearings. A jet wash is a big no-no as the pressure could blast out this vital grease.
Once your bike is wet and clean enough to actually identify the individual parts, give it a good spray down with bike cleaner, working from the top of the bike to the ground. Leave it to soak and penetrate the dirt for a minute or two.
Using a brush or sponge, scrub your bike, again working from the top. This means you tackle the cleanest parts before getting to the really grubby bits lower down - the wheels, tyres and drivetrain. Give it a good rinse with buckets of water or the hose, again beginning at the top and working down.
Time to get to the nitty gritty - and if you've been riding for a while since your last wash, your bike's gears and chain will be very gritty indeed.
After removing the worst of the crud with bike cleaner, drop the chain off the cassette and chainrings and spray the rear cassette, front chainrings and the chain with a good quality degreaser. Leave it to do its thing for a few minutes.
You can buy narrow brushes for flossing between the teeth of your bike's cogs and chainrings, but an old toothbrush does the job too.
To really give the chain a thorough clean, you can remove it from the bike completely if it has a quick-release link. Fill an old bike bottle with degreaser, put the chain in and give it a shake before rinsing it thoroughly. You can also give your chain a good clean while leaving it on the bike if you use a chain cleaning machine - fill it with degreaser and follow the instructions for running your chain through it.
Once the drivetrain is spotless, go over your bike once more with bike cleaner to remove the last traces of dirt, plus any grease and gunk which have sprayed from the drivetrain onto the frame.
Allow your bike to dry thoroughly, or use dry rags or a chamois to remove water. Put the chain back on if you removed it to get it clean.
Apply bike-specific lube to your chain sparingly. Wind the pedals backwards and run the bottle along the chain for even coverage. Wipe off excess with a dry rag.
Finally, a bike polish spray will give your frame a lustrous shine. It'll also help keep it cleaner for longer by stopping mud from sticking. Take care to avoid brakes, especially disc brakes, when you spray it as it can stop them from working properly.