The SM57 is a midrangey mic that will probably emphasize an already nasal voice. If your nose is stopped-up, try those breathe-right nasal strips that you see atheletes wearing. They tend to open up the nasal passages a bit.
Also, work on mic positioning. Put the diaphragm of the mic about eye level, angled down toward your mouth. This will force you to open up your throat a little and may minimize the nasally-ness.
For occasional use only, you might try one of those vicks inhalers. I'd avoid Dristan and the like, and only use the vicks periodically (no more than a couple of times a day for 2-3 days, maximum), as they can lead to physiologic dependence, actually worsening your problem.