I've encountered a few cat abscesses, the first was on the top of my 17 year old tom cat's head. As he was very poorly and off his food we took him to the vet. The vet gave home some antibiotics and decided to take him out the back to try bursting it himself. The abscess was clearly not ready and really distressed the cat. He was unsuccessful. After paying a small fortune we took home a poorly cat who felt worse than when we took him. A day or two later, the abscess began to burst by itself, probably with the aid of the Cat cleaning he's head. We helped the old tom to get the remaining pus out. The following day, he was right as rain, although he looked a bald mess.
The next abscess I encountered was on my 1 year old kitten. He came home one day with a lump the size of a golf ball poking out from behind his ear. Initially I thought it was a bee sting, but two days later it was still there. Being as he was only a year old and must have a good immune system he was still eating and drinking, although he wasn't as playful as his usual little self. Once I realised that this would be another abscess I was quite reluctant to take him to the vet as I remembered clearly how useless the vet was with my old tom.
Sure enough, the fur started to full out around the affected area to reveal a monstrosity of an abscess! Once revealed, I began to soften it by soaking it in warm salty water a few times a day. Another day or two later (Keeping in mind my cat was still eating) I found him 3 in the morning frantically cleaning his fur, the abscess had begin to burst.
Time for the two man job, we took little Zak into the bathroom, my partner held him whilst I gently squeezed about 2-3 table spoons of pus from this wound! I never seen an abscess like this! Once all out, there remained a hole big enough to poke your finger in. Through out the delicate operation, I wore rubber gloves and used some more warm salty solution and a clean cloth. He looked and absolute state but he brightened up by morning and was back to chasing butterflies in the garden. I kept soaking the wound in warm salty water a few times a day. Throughout this whole time, the cat was very good and let me do what I needed to do without any fuss, however some cats aren't as trusting, so you may need to wrap them in a towel to prevent you from being clawed. Try not to let the scab form as this can trap in the bacteria. Fortunately Zak kept scratching it off himself as it was healing. His face has healed beautifully now and there was no need for any traumatic, expensive vet visits. As an owner though, you do need to judge whether your cat needs to visit the vets. This was just my experience, and remember my cat didn't loose his appetite, which was less cause for alarm.
I hope this may help someone else and also save them an expensive vet bill.