Nick sums it up pretty well. I only have a few things to add...
I do not use a RAID array. To back up, I have a copy of the raw files and mastered flacs on both external HD and data DVDs, and then an extra copy on either data DVD or external HD of the FLACs and/or raw files living at another location (a responsible band member's house).
It's been said that your data is not important if it is not stored in 3 different places. I'll add that one of those back-ups should be in a different physical location in the event of fire/flood, etc. LMA can be great for this, but only for final .flac files.
Make clear with the band at the outset who owns the masters. Part of my arrangement with the bands I archive is that I individually retain ownership of my masters/master files (as the band's 'live recordist'), and I get a percentage out of anything used commercially that is produced from my work. Anything that is not 'stellar' and potentially being considered for official release gets uploaded to the LMA. Under such an agreement, the band understands that I am also making an investment in them and their future and feels that I am more "a part of the team."
I also recommend coming to some agreement on the principle that you should be reimbursed for all duly accounted for expenses associated with your archiving. You will be bringing your gear, experience, and time commitment to the table which is a very significant benefit to a band, and it is not unreasonable to request reimbursement, for instance, for a dedicated external HD for their stuff (for 2nd or 3rd back up, to be kept at one of their houses when not being filled by you), a spindle of blanks to burn them copies, gas money to travel to shows outside of the local area, etc.
It is also good to have a proper sit down with the band and management re: what authority you have to respresent/speak on behalf of/act for the band in the capacity you will fulfill for them. Otherwise, it may be easy to get roped into other duties you do not want or, to the contrary, to not feel like you have sufficient authority to approach venue staff as 'band crew' vs. a 'fan.'
Weekday shows can be rough. To avoid any hurt feelings or different expectations between the band and yourself, it is also a good idea to be clear at the outset that some shows are going to be too difficult for you to attend and they should be understanding of that and not expect you to sacrifice family, financial well-being, and (too much) sleep.
Along with the perks Nick mentions, there is also the offer that sometimes comes up from openers/venues/other bands to record them for pay. Being in an 'official' position with the band you're working with gives you an additional level of credibility and respect with venues and other artists that opens up such opportunities that may not have generally been present.