Well as Mike mentioned, he's a very close neighbor to me so the few times he's asked me pop in on his tank and feed (frozen) it's not been a problem. He trusted me enough to give me a key to his house which is where the "you have to really know the person" comes into play as opposed to just asking someone to come by and do it. He even had me deal with his RO water make up (filling a very large reservoir) and when I do feed I do take a look at his sump and what not just to get a quick visual to see if anything is out of the ordinary. One such instance I noticed one (or two) vortechs were on the bottom not connected however rather than just fixing it I sent him a text to see if this is normal (it was). I have yet to get him to return the favor, mostly because my fish tank is "very" temporary to the point of fish food is the only thing that it needs and an autofeeder takes care of it, but if I had a nice array of corals and what not I absolutely would hit him up as opposed to relying on family members. Here's why
about 10 or so years ago, I did have a nice tank with corals, and I went away to Hawaii for 10 days. So I needed him to 1, feed my cats, and 2, deal with my tank. The cats were easy, but my father being my father found a way to screw that up by not looking in the cabinet with the food that I put a big sign on that said "CAT FOOD IN HERE". Then came the tank, I kept things simple, I wasn't dosing, the only thing I really worried about was food and evaporation (this was pre-ATO days), I made this check list of things to look at, detailed yet simplistic instructions of what to look for and even a chart to check off everything just to give some queue that everything has been done. All the food was put into tiny cups, one cup a day, the check list was all checked off but it looked like it was all checked off at one point with my father and his "I'm not going to do this crap" mentality, and the top off water consisted of a small bucket, to grab water from a big brute full of RO/DI water. There was tape on the sump (which was raised and not under the tank) which labeled both low level "fill me up" and high level "fill me up to here". Well I got home to that burnt electrical smell, and found my power cords were melted in the strip due to water infiltration, now can I prove it was him? No... but again... knowing my father.
So bottom line
1) Know the person who's going to look after your tank, make sure they are capable of following instructions you leave (my father wasn't), and not just doing anything the way they want.
2) Make sure the person can get in contact with you when you're away (again my father wasn't, as he has no cell phone)
3) KISS, make sure what you need is a reduced tank maintenance schedule. Make things automatic, make sure that it can easily be corrected or shut off if something goes wrong, and be prepared to do a little extra TLC on your tank.
4) The longer you're away the more chances are things can go wrong even with someone looking in on the tank. Throw a reef tank into the same category as dog or something, in that it limits you from going away for 2-3 week long vacations, especially if you have a "sensitive" tank (i.e. SPS heavy)