Bad speakers = bad mix.
If music means that much to you, go invest in a real set of monitors. If it's a hobby, then don't expect pro results from bad equipment.
You technically need a treated room. Technically, you need a room that's BUILT SPECIFICALLY for mastering, so the size/ratios are good.
If you can't do-- which I'm taking you can't-- that get a set of Mackie HR624's or 824's and set them (with the switch on the back) to the room size similar to your room.
NEVER believe that these are the final solution, though.
Have another set of those Yamaha monitors that everyone and their moms use to A/B.
Then always test to see how it sounds in your:
- car
- on computer speakers
- those tiny CD boomboxes
- every imaginable sonic output.
Notice what "changes" in the mix. Also, listening on different speakers-- though common in mastering-- helps your ears "switch" to hear different sounds or "areas" and "empty spaces" you can exploit in the mix.
Listen to the way the mix sounds outside of the studio, too. Stand in the hallway.
NEVER use headphones to mix a track. I repeat: NEVER use headphones as the final reference.