I have headed off mastitis twice - one right after Ben was born and once just a few weeks ago after being exhasted from visiting family in the UK. Each time it had progressed to the point where I was achy, had a fever, and was experiencing chills. This is what I did:
1) As PP say, REST! Go straight to bed if you can. This second time, I had to go to work for a short time one day, but just try to stay in bed.
2) I didn't use 'sposies (don't have any in the house!) but I did take two short baths a day where the water came up high enough to cover my breast. During and after each bath, I expressed as much milk as I could
3) Nursed Ben mostly out of the affected side. I didn't ignore the other side (it's my lower producer anyway) but you have to get the milk out of the breast to heal. After he nursed each time I would express until I was only getting little dribbles and no streams. You can deal with any resultant over-supply later.
4) I used ibuprofen to combat inflamation and (less importantly) reduce fever. In each case, I only needed one or two doses before I really began to feel much better.
In each case, it took about 48 hours before I felt like I was really recoverd, and much of that recovery took place in the last 12 hours. For the first 36, I felt pretty constantly crappy. If my symptoms had persisted longer than 3 whole days, or if I had felt like I was getting worse or still had a high fever after taking ibuprofen, I would've gone to the dr.