
Your guinea pig must be completely dry before he is allowed to continue his day. This is especially important if your guinea pig lives outside or in shed that has no heating.
Once rinsed, wrap your guinea pig in a towel, find somewhere comfortable to sit (in front of the TV usually works for me), and spend some time gently rubbing him down and drying him off. He may start to purr (vibrate) if he is enjoying the attention.
Some guinea pigs enjoy being dried with a hairdryer. If you choose to try this method, use the lowest heat setting and hold the hairdryer at a safe distance from your pet so you don't burn him. Lots of guinea pigs don't like the noise of a hairdryer, so this method is not suitable for everyone.
When your guinea pig is no longer wet and is just damp, swap to your other towel which should be completely dry and continue to rub him down with that.
This is a good time to do some grooming and some nail clipping.

Above: a clean grease gland
Male guinea pigs have a grease gland that is located on the rear end above the testicals. The grease gland regularly produces grease and this needs to be cleaned every so often or the grease will form a scab, and the skin underneath will become sore and sensitive.
To clean the grease gland, you need a tub of Swarfega gel (used by mechanics to remove grease and available from DIY stores), a towel, a bowl of warm water, and a comb with fine teeth.
Wet the grease gland with water and then rub in some Swarfega, before leaving for a couple of minutes to soften the grease. Rinse the Swarfega from the guinea pig and then gently use the comb to pick away the greasy scab. The skin underneath may be sore and sensitive if the grease has been allowed to build up, so once the grease has been removed, dry the guinea pig and apply some Kamillosan cream to sooth the skin.