Amplifiers contain voltages high enough to Kill you even after the amp has been powered Off !!! If you don't have enough electronic experience to identify and discharge the Filter Capacitors which contain these Deadly voltages, take your amp to a Qualified tech. If you don't know a qualified tech in your area, post a request in the alt.guitar.amps newsgroup, the folks there will be happy to help you.
1. Changing all the 12AX7's plate resistors to 100K's except the Phase Invertor & Reverb tube. This will make the amp more "touch-friendly" and more Marshall'esq. This will also take the "buzzy" tone out of the amp. It does reduce the gain so if you want more gain see #2 and #3.
2. Locate the 10K cathode resistor (R13 found on V2) and place a .68uf cap across it. This will increase the gain Alot!. It also boosts the mids nicely to get that sweet Marshall "crunch". If you're into classic rock, you may not want this cap on there. If you like "heavier-tones" then run with it.
3. Locate 1.5K cathode resistor (R16 found on V2) and replace it with an 820ohm. This will increase the gain slighty and make the amp a bit more responsive (depending on if your amp matches one of the two 2205/4210 schematics avalible)