Nobody likes to spend money on monitor speakers. It drives them crazy, especially if they already have a stereo system, just spent all their money on a 4-track, etc. and so forth. Eventually you will have to do it, but using some of these suggestions, tips, and tricks of the trade might help you stave off that day to the future.
One suggestion, endorsed by almost everyone, is to listen to your test mix in a number of environments before you finalize it, such as a car, a boom box, and a home stereo system.
My personal "trick" here saves a bit of time...I have a set of boom box speakers hooked to my monitor amplifier by a switch. One flip, and I can get a rough idea of how the mix sounds on a boom box (although nothing beats the real thing :-).
Another good tip (which I rarely follow myself, but perhaps writing this will shame me into it) is to mix the "morning after" the recording session is done, when you haven't been listening to the same material over and over already. This gives your ears a chance to recover.
A variation on this technique is to make a test mix at night, then listen to it in the morning. Experienced practitioners of this technique never fail to be shocked at just how badly they were mxing the night before (usually, a sign of this problem is realizing that you had added too much reverb), and this saves making copies of an embarrassing mix and then having to get them all back from your friends. :-)
As the above indicates, a large part of mixing is psychological in nature;
you're using both halves of your brain pretty heavily. While the "mechanical"
aspects of twiddling knobs are important, mental and audio fatigue can
ruin a mix as much as a botched fade...and can be more subtle and hard
to detect. After all, who's to know how great the mix could have been,
if you hadn't been in such a hurry to finish it that you didn't even think
of making one last EQ adjustment?