Major things to consider:
1) If you know you are going to be gone for a long time, PLAN AHEAD. Don't spawn fish and have tiny, vulnerable fry in your tanks right before you leave! (Unless it's unavoidable/unplanned, then you have no choice). If someone is looking after your fish, all fish should be able to eat the easiest to measure food possible, like dry pellets with a measuring spoon. Put fish in the largest volume tanks they can be in with tankmates that you know will be peaceful pairings. The larger the volume = the more stable the water parameters.
2) Make the last major changes at least 2 days before you leave. This gives your fish time to settle down so you can observe their real behavior before you go. If someone is taking care of your fish, you can accurately measure out exactly how much you want them to feed.
3) Water changes at least a day before you leave. These don't have to be major ones. Again, you need at least a day so you can observe your fish.
4) Don't overfeed the fish! If you have been taking care of your fish well, then they don't need any help surviving a week without food!
5) If someone is taking care of your fish, leave EXPLICIT, SPECIFIC instructions on both paper AND on the tanks. I use bright yellow tags as tank ID's and blue painter's tape for instructions so that everything stands out very easily.
6) Put everything on timers. Seriously, it makes everything so much easier (even when you aren't on vacation).
Some people recommend dimming the lights, shortening the light cycle, cooling the temperature, etc. etc. I like to keep everything exactly the same. It's up to you if you want to experiment trying to slow down their metabolism. I just find it easier not to change anything that I don't absolutely have to.
Happy Holidays Everyone!