Believe me when I tell you, save up the scratch to get some good quality studio monitors. You want a monitor that will give you a true, uncolored, flat response so that when you're mixing, your mixes will translate well on just about any speaker or headphone you play them on. Your monitors are the one piece of gear you have in your studio that you can actually hear so even if you have a $150K SSL board, Protools HD system with the best converters money can buy, Nuemann mics, Vintage Neve compressors and EQs, ect. if you're mixing out of inferior monitors, none of that will mean shit.
Some things to consider when selecting a monitor.
Genre of the music you will be mixing on them: (If you produce or mix mostly urban music, then you'll want a monitor that will translate the low end properly for you. If you're mixing acoustic music or spoken word, a monitor with detailed mids would be ideal). Best bet is to go with a monitor that does both fairly well.
Size of the room you will be mixing in: (Big 8" woofer monitors in a small room will be over bearing and won't give you a true representation of your mix). If you need low end, sometimes it's better to pair a sub woofer with some 5" to 6" monitors.
Brand and reputation of the manufacture: (It seems like this wouldn't be important, but it is). Take for example, the famous Yamaha NS-10s...those things sound like shit yet every major studio and engineer has a set that they use as their preferred near field monitor. Why, you ask? It's because if you can make your mix sound good on those things, your mix will sound good coming out of any speakers. Think about all the hit songs that have been mixed through NS-10s? As much as I hate them, KRKs are highly used by reputable producers in the hip-hop genre. They are not the best speaker that money can buy but, they are a standard for that genre of music. Genelecs are a known and reputable staple in high end studios but they cost. Again, now that studio monitor technology is getting better, Genelecs are not necessarily the best choice but considering how many hit records were mixed on them, you can't sleep on them if you can afford them.
Listening pleasure: Some monitors even though they may have flat response have a harshness to their sound that may make listening to them for hours on end, fatiguing. I think it's important that your monitors feel comfortable to your ears and that you actually just enjoy listening to music on them. They should have a nice "sweet spot" that makes the sound clear and nice without adding color.
Test drive them: Honestly, your ears don't lie (unless you are tone deaf). Go to the music store and listen to all the monitors. Bring in one of your own mixes and a mix from a song that you like that is similar in composition to that song and listen to them both. A good pair of monitors should immediately expose the deficiencies in your mix, if you hear none then stay with the monitors you already have.
