One thing you could try is mixing into a compressor that you've set up on the master fader. So, before you even start mixing, put a compressor on that (you could add it after you've started a mix, but you might find that it changes the sound of your mix quite a bit, so that's the only reason to put that on first... that way you won't have to re-adjust levels or plugin settings. I would recommend doing this with a nice sounding compressor though, something like the Waves SSL compressor is good for it. Quite a few people use the API 2500 plugin for this. There's loads of good compressors out there, it's up to you what you use. It helps with the mix, because every track in the mix is benefitting from the same compression (you probably don't want to be hitting the compressor too hard, and killing your mix; that's something to be aware of) which brings it all together.
Also, console emulations. Slate VCC is absolutely brilliant for giving the whole mix a bit of consistency and character. It's an amazing-sounding plugin and I totally recommend it. Another one is Waves NLS which is good, but I prefer Slate's one.
Try tape plugins too. Tape saturation can provide a nice 'glue' for a mix. Waves Kramer Tape is a good one, UAD also have a Studer A800 model which is supposed to be good. Slate also make one called VTM which has gotten rave reviews, and I'm going to be buying that soon.
Trying all three of the things above will really help bring your mix together, but you can also try things like setting up buses for all the different instruments, so you'll have a drum bus, a vocal bus etc. That way all of your drums will be getting compressed by the same compressor, all of your vocals will run through the same EQ or whatever. If tracks are consistent with the effects applied to them, it'll help bring the mix together. Of course, it's better to use things on buses, and on the stereo bus, with a degree of subtlety. You want your tracks to all move in the same direction, but you also want to maintain that element of individuality about all the tracks. So keep all the various different compressors and EQs on the individual tracks, and have the effects on buses just doing a tiny amount.
For filling out your mixes, use the obvious effects like reverbs and delays. But also use things like stereo width plugins (these can be good on synth pads). Distortion, which can be useful if you want to give your bass tracks a little more character in higher frequencies. Make use of panning, you don't have to have everything coming out of both speakers all of the time. Also, if you've got a lot of space in your mix, and it's very sparse, just mix certain elements more aggressively. Take advantage of the extra space to make something bigger.