Unless you've been operating your recording studio from under a rock since 1965 (or are one of those curmudgeonly "haters") you've probably got an SM57 or SM58 in your mic locker. With their screw-open bodies, these mics are exceedingly simple and rewarding to mod.
Removing the Transformer (the "TapeOp Mod")
The stock transformer in the SM57 increases the output level by about 12dB, but it also contributes to the 57's somewhat pinched and midrangey sound. Taking it out is a quick and free way to increase the frequency and transient response of your mic. The video below walks you through the steps for this mod:
TAB-Funkenwerk Transformer Swap
TAB-Funkenwerk, maker of fine vintage condenser microphone kits, manufactures a replacement for the stock SM57/58 transformer. The T58 transformer is designed to maintain the basic SM57 sound while extending the high and low frequency response. T58s are available from for $80 each.
The Impedance Mod
Nowadays, most mic preamps and mixers present a rather high input impedance for condenser microphones. However, the SM57 was designed in the days of the standard 600ohm input and theoretically performs better when more heavily loaded (lower input impedance). By connecting a resistor across the preamp's input, we can increase the load on the SM57. In his Recording Magazine article (link below), Paul Stamler provides the following formula for calculating the resistor, where Rg is resistor to be strapped across the input, Rd is the desired input impedance, and Ra is the actual impedance of the preamp or mixer: 1/Rg = 1/Zd – 1/Za. The mod can be built into the male plug of an xlr cable or an xlr barrel connector by soldering a resistor between pins 2 and 3. Alternatively a simple box with a 1k linear resistor would provide a good range of impedances for different inputs.