Greetings!
I recorded a whole bunch of
concerts with a stereo mic into a minidisk recorder. The sound was not bad, but there are a few things you need to watch for:
a) Get the best mic you can. The hat mounted mics are surprisingly good. I had a full sized mic on a mic stand and got passable results. Some of the smaller mics are actually better.
b) Find a spot away from loud crowd noise or speakers. If possible, get into a taping section or let those around you know you are recording. This way, people are a little more respectful of the recording and don't start talking / shouting / singing along mid song.
c) Turn off any compression mechanisms on the recorder! Some small recording devices have a really evil feature of an automatic compressor. Think of it as a poor imitation / implementation of VolAdjust. This recording is a one of a kind, one time master - it is easy to compress later, but really hard to edit out after the fact. And it is intensely annoying.
d) Spare batteries and disks are always a good idea.
e) Depending on the venue, sound board recordings can be better or worse. For small scale (bar, club, coffeehouse) think about what is actually being amplified. In most cases, drums or loud acoustic instruments are not amplified and do not go through the soundboard. The sound may be great in the venue, but listening later will show all of the acoustic / unamplified material missing.
e) If possible stereo recording is nice - with a stereo mic. Do not settle for splitting channels if you can avoid it. If you only have a mono mic, see if you can get one channel from the soundboard, one from the mic. This gives you the ability to mix it later.
f) Mixing and mastering after the fact is always a good thing. Clean up anything that seems strange. With smaller bands / venues, there tends to be a lot more filler for tuning, setup between songs, etc. Some of that space can be edited. Some of it can also be really funny, depending on the band.
g) Watch for clipping! Sound checks are a good idea if possible, but the levels always change when the room is filled with people. Also, many bands that I know start off kind of slowly, quietly, building to a crescendo that will clip if you are not ready for it.
h) Pay attention. I know you are also there to enjoy the concert, but I have never thought of live recording to be a "set it and forget it" type thing. Especially if I want good results later on. Keep an eye on the input levels (don't play with them unless you have to), battery and time remaining (whatever media you use).
i) Oh, if you have permission from the band, do not actually attach a hat mic to your hat. Find an immobile object to attach it to, preferably not a wall or sound reflective item. Moving your head will change the spacial feel a bit, especially on a stereo mic!