It's very easy to pile up a lot of effects on your tracks, trying to get each track to sound it's best. However it's also easy for it to get out of control. Too many reverb tails bouncing around, too much mud, no life, no coherence.
Here's a few good ideas on plugin placement.

Reverb-When using reverb select your best reverb plugin and put it in AUX1, then feed each track to it in varying amounts. This will keep the whole recording in a common setting and stop the problem of a lot of varying reverb tails muddying up your mix. Need a seperate reverb for the vocals than for the drums? That's OK- there are 8 AUX slots. Use one for drums and one for vocals if you need to, but watch out for having too many reverberations bouncing around muddying up your mix. Also using a high pass setting after the reverb can help keep those lower frequencies from bouncing around (they cause mud quicker). To do this just pur an EQ after the reverb and cut the low end.

Exciter- Another good plugin to put in the Aux slots- you want to 'add' some life to the sound, not effect the whole sound, so the AUX area is great for this because we can feed each track a varying amount. Send the cymals there for more sizzle, send the vocal there to get more 's' and 't', etc.

Compression- You'll end up using compression on a lot of tracks singly. An AUX slot is the wrong place for a compressor, unless it is used to enhance another effect, such as after a reverb unit to really bring the reverb out on a vocal track, etc. You don't want to feed multiple tracks to the same compressor as a rule, they will muddy up quick. The exception is when using a multiband compressor on the mix, or as a limiter to control a group of tracks.

EQ is best done on individual tracks when mixing. It is a string tool when mastering too, but during the mixing stage it is best to EQ each track as efficiently as possible. Cut the frequencies that are not necessary for the track to keep noise levels down. These small noise levels don't seem apparent now, but later, when mastering, they can rear their ugly head under the stress of a compressor/limiter designed to really bring out the sound. Example- on a guitar track you can safely cut from 100Hz on down, and it's common to be able to cut from higher than that without affecting the percieved sound of the track, and this leaves that all important 'room' for another track.

Realtime Analyzer- the best place for this is in the final slot of the Main Output FX area. When this is in that slot you can easily check the frequencies of any track by soloing it (to aid in seeing what you can EQ out,etc.) and also check your mix overall for level, holes in the frequency spectrum, etc.
If you use WindowsMediaPlayer it has a display called 'bars' that is very much like the RTA in Powertracks. Using this when listening to CD's will help train the eye to 'see' what you hear in a mix. This can help to solve those mysterious mix problems when 'somthing doesn't quite sound right'.
A big notch in a certain area signals a hole in the mix, too much bass becomes easy to see and identify.

PGPeakLimit- One of the most used plugins here. Nothing goes out of my main outs without passing thru this. I put it in the second to last slot of the Main Outputs effects area so NOTHING clips on it's way out. With this and the RTA I know exactly how loud my mix is.

Usually the first two slots in the Main output are left blank when mixing down, but for mastering..
I keep the RTA and the PGPeakLimit right where they are all the time; the last two slots before sound leaves PT. In the first slot will usually be a multiband compressor (Grancomp,T-Sledge and Triplecomp are great freebies) and in the second slot is a master limiter like ClassicMasterLimiter.
The signal goes 'multicompressor-masterlimiter-peaklimit-rta' as it goes thru the main outs.
When mastering a wavefle this still leaves the four effects slots in the track area for EQ, the enhancer and reverb can still go into the AUX area and you have plenty of room and power in Powertracks for mastering.

I do not work here, but the benefits are still awesome
Make your sound your own!