Quickest tip that'll give you somewhere to start from...

If you have two parts that are clashing with each other (say, you can hear your lead fine, but when the Big Epic Pad (TM) comes in, the lead seems to get buried unless you have the pad really quiet), you need to work out what is making the parts tick.

In the case of a pad, it will wash across a large frequency range, cos that's what it does. Pad the mix out. Hence the name ;) A lead, on the other hand, will usually have a fairly confined active frequency range where it does its shit. So basically what you're getting is that the pad is doing too much in the same frequency range as the lead.

Run an EQ in line with the lead, start at the lowest frequency and quite a high Q, and a fairly sizey dB boost.

Gradually raise the frequency and listen to the sound change. Generally at some point the "main" bit of the lead will suddenly jump out at you. move it around this region and you'll be able to demarkate the lead's main frequency band.

Make a note of this, bypass or disconnect the EQ, then run an EQ in line with the pad.

Apply a bell cut around the frequency range where the lead lives, so that the pad no longer has much presence there.

Fiddle around with this a little more and you'll siddenly find that the lead and pad "fit" with each other far better. You can apply this method of working (with varioations) in most circumstances.