1. Warping Tunes is Easy
You don't need to go through and adjust every single drumbeat in a track in order to get a tight mix in Ableton, The best method i've found is as follows. Firstly, tell Ableton where the start of the material you want to warp is. That means moving your marker to the start of the first kick kick drum in the song. Next, right click and select the "Set 1.1.1 here..." option, then right click on the marker again and select "Warp from here straight..." option. This is the best way to warp transient material like drums so that they lock to a certain tempo. However you'll notice that the warping starts to drift out towards the second half of the track. To remedy this, grab your loop brace and resize it to 4 bars. then using the arrow keys move it up and down, all the while warping the kick drum at the start of every four bars by inserting a marker and dragging it in line with the grid and the loop brace. Below is a video that explains a bit more and gives you other advice on warping, but essentially it should only take a couple of minutes each track and no longer.
2. Harmonic Mixing is a Must
Get yourself a copy of Mixed in key and ensure that you're mixing in a way that makes musical sense. on't just throw two tracks together because you like them, they might combine in a way that produces a horrible clash of frequencies. Make sure that each of the tracks you are blending together are harmonically related. The best way that i've found to do this is by naming the file with the key in it, and colour coding different keys in Ableton. So, i'll have a file called something like [F Minor] Niwun - Alone.wav and this might be coloured yellow along with all the other F Minor tracks in my set. I then place all of the tracks that are in the same key in horizontal layers so that I can move through my set in a way that makes musical sense. When a key change happens I can make sure that the keys are harmonically related, then I can mix within a key by staying within that layer and that colour.
3. Flexibility is Important
Preparation is vital when you are performing live, however you cant overprepapre to the point where you are rigid in your tune order and selection and can't adapt. What happens if you get behind the decks and the first three tunes that you play absolutely bomb? What if you realise you now need a different style of music to retain the crowd's interest? The way I get around this is by tagging the mp3 file of all the warped tunes I have on my laptop. I tag them by entering genre descriptive words in the description section, using a program like IDv3 Tag or Mp3 Tag that lets me edit the IDv2 and IDv3 information in the mp3 file. So in the description section i'll put words like "Minimal, Funky, Soulful, Hard, Dark..." and the like. If i need to quickly bring in some music into the set on the fly, I can do a spotlight search on my Mac by pressing command and the spacebar. If I have decided that this party needs something minimal, I just type this into the spotlight search and then whatever files i've tagged with this information will come up in the search menu. I drag from the menu directly into Ableton and a prewarped clip will appear, one that maybe wast originally in your set list for the night.
4. Ensure the Best Sound Quality Possible
This should go without saying. If you're using mp3's then use 320kbps versions, most places like Beatport or Juno Download will provide digital files in this and other formats. Don't rip a music video off Youtube and expect it to sound good in a mix, it won't. Also important is the warping algorithm you choose in Ableton as this has the potential to also affect the sound quality. Generally I go for the repitch algorithm as long as there is no audible change in putch to any of the melodies in the song. This algorithm works like adjusting the pitch shifter on a turntable and will affect the pitch as a result. Be wary of this if you are using harmonic mixing, if you move a track away from it's original speed by more than 6% then you are changing it's key. Thats why I would err on the side of caution and not use this algorithm if you can hear an audible change in pitch. In that case I would go for the beats algorithm which works in the same way a sampler does, by chopping everything into slices and replaying it that way. If you are getting a glitching sound in the beats mode, then and only then go to complex or complex pro, as these algorithms are designed for instruments and the human voice, and they will most definitely affect the sound quality of the mp3 or other digital file you are using.
5. Use Effects and Practice
Ableton removes the need to beatmatch when mixing. You have a lot more time to do ther things, and while you still need to focus on using your DJ Mixer or other controller and it's 3 band EQ, i'd suggest that you set up some cool sound effects and use these to add interest to your mix. I use a few, including a flanger, a beat repeat device, and a metal tube effect. Don't go overbard though, let the music also speak for itself. Also you need to practice your routine as much as possible. Learn how to control the dynamics of the mix with your DJ Mixer or controller. Know your material, and know your mix points in each song. If you practice enough you'll put in a decent performance, and provided that you have done all you can to get pople through the door, hopefully the promoter will book you again.