What do you consider when setting crossover freqs?

Lot's of good theories here, including Chadlinz's input. It depends, though. In a perfect acoustic space and setup, you could find the 3db down point of the mains, and ajust the sub to mate. However there are other variables.
A main variable is that typical receiver and passive speaker combo's opperate much more efficiently - regardless of response potential of the mains - crossed over to a powered subwoofer at a certain frequency range. THX finds that at about 80hz. Set up well, I find that a superb location, personally.
Basically, even if your mains can play down to, say, 45 hz, I know from experience that you can get my better dynamics, dynamic range and power handling if you cross em over at 80hz, and let the active sub woofer handle the dirty dubties. The receiver (being week on current, relative to separates amps) needs all the dynamic help it can! Also, the typical medium sensitivity passive speaker also needs much dynamic help, as it can't control the drivers half as efficiently as an active configuration.
Another consideration, as hinted at here above by others, is that the room is a huge factor. You might find, for instance, that the best location - all factors considered - for the speakers might be where they have a down point, or even hole at a certain frequency, where they'd function better crossed over elsewhere. So it depends. Same can be said about the subwoofer, which needs care in placement.
So, it's a balancing act, with lots of variables to consider. Basically, there's no "this is THE BEST setting, er whatever, across the board.
Hope this helps

usually 10hz below the main bottom response so the blending does not leave a bass hump - start low and move up til you hear it and then back down a notch, is a very basic STARTING point

when all else fails try the two pink things then engage grey matter follow with bubbly

For Bass, set it by ear. In my unusual system where the custom subwoofers are good to 350 or so and the mains are MG1.6, good to 40 Hz, the best setting depends on the music. I have a convenient way to change the frequency (a knob).

One problem is that the bass measurement changes when you change seats. If you are closer to a wall or corner, you will measure, hear, and feel more bass. You get it all figured out, but then move to the other end of the couch, and you might feel like you need to make adjustments. I think that is why people get more than one sub- it helps even things out a little bit. But then you have to balance your mains and 2 subs. AAAAAAAH! Just make sure you sit in the same place.

Girst I loook at specs for ballpark then adjust to my taste, the room is gonna be a big factor too. Its really gonna come down to material aswell.

You are dealing with 2 curves and the idea is to blend them so that the result is a flat line. Not possible but that is what you are aiming for. For example, if the main speaker is -3db at 60Hz you would set the subs crossover at a point that would boost 60hz output back to flat. Lacking sophisticated measuring equipment (which is getting cheaper all the time) the best thing to do is set it by ear, as all rooms will boost or suck out the bass to some degree. Get a CD or LP with test tones at different frequencies and set the crossovers to produce the same level of sound at the different frequencies. Some subs have complex circuitry to do this for you but you can do it for yourself, it takes longer.

Depends on the active/passive implementation.I usually look at the response of the "main" speaker, stated at its' 3db down point.If using a passive cap/inductor arrangement,then you are looking at an octave up or down +/- (twice the Freq going up/half going down-will be an octave).This will also depend on the crossover slope (6,12,18,24 in db/octave).^ being gradual and 24 being steep.Some active units allow up to 90db slopes.