Well guys you're lucky. Dropmixing(=slam mixing or slipcueing) is one of the many tricks Tricky and me put in the mixing guide (it's on the way). But just to give you a look I'll post the dropmix section here (this is about 1/20 of the complete guide!)
DROP MIXING
"What, do I have to drop my records too to mix?", I hear you think. No don't worry, it's nothing like that. Dropmixing is the art of dropping out of a tune right into another, without missing a beat. How do you do that? Normally, you'll use the xfader as a switch to cut from one tune into the other. But it's not that easy. It's very important to choose the right moment to perform this trick in your set, because don't play both tunes in the mix at the same time. Good timing is essential, timing in both meanings : the timing of the dropmix and the moment you perform it. Needless to say that if, for example, you perform the drop in the middle of a sentence of the vocalist, it will sound incoherent. Also dropping in the right tune at the right moment is very important, but that's something you'll have to learn with experience.
How do you perform a good dropmix? You monitor the record you want to bring in in your phones. Find a good starting point on this record. (for example a drumfill or the first beat of the first bar of the tune). Like always be sure that the channel of the according record isn't open, or the crowd will hear the sound of you searching the starting point!!!
You can play the start sound repeatedly by cueing up the record to that point and releasing the vinyl, so you can hear how it will sound and you can adjust the general volume levels. Remember that releasing the vinyl is a bad term. Like you would do normally when starting the record for beatmatching, you've got to throw it (give it a little push).
As said above, you have to adjust the volume level so they are both almost equally loud (again this is for the coherence of the mix). Just do this buy adjusting the gain/trim.
When you are ready to perform the dropmix, open the fader of the record you want to bring in (nothing as embarassing as performing a dropmix, and no sound coming out of the speakers, when you cut away the first tune!!!). Be sure the xfader is on the side of the old track.
When you release(=push) the new record, flick the xfader over so you cut from the old tune to the new one. Try to do this so the new track falls in time with what has been played before. For example if the start point of the new track is the first beat of a phrase, wait until the end of a 4, 8, 16 or 32 bar phrase of the old track, to do the drop. That way the first beat of the new tune will feel like the start of a new phrase.
A lot of dj's find it easier to time the drop perfectly when they do a simple baby scratch (moving the record back and forth) on the rhythm of the last four beats in the last bar of the phrase, before you drop in the new track. It helps feel where you have to let go. A little hint : when you are comfortable doing that baby scratch on the rhythm of the old tune, you can bring to xfader over a little bit earlier so that scratch will be heard in the crowd (it's a nice effect, but be sure the scratches are on the spot).
Normally in the guide there will be a small sound sample to make things clear, but still have to record them
I think of a bar as old forty year olds drinking to get away from there wife