Rollo Tomassi
Master Don Juan
The most important technique I think a guy can master is developing an ability to do cold reads. PUA skills are all useful tools and can be applied in a variety of settings, but being able to "read" your target will improve any other PUA techniques you apply. Once you've 'read' your mark you'll have a better idea of what will or wont be useful in a given set.
Most aspiring PUAs read up on technique, learn a few scripts, and indiscriminately fumble into a set without concern that they're simply barking up the wrong tree or use some pre-packaged C&F that may have worked for some other PUA in a completely different environment with a completely different set of conditions with regard to the girl. What may work on an HB 9 at a high end martini bar wont play with an HB 8 at a Slipknot concert.
That's an easy illustration. Obviously different approaches are warranted for different situations, but reading subtleties and looking for cues with a good read when you see a woman you'd like to approach or one who's giving you IOIs is important. A lot of mPUAs like to promote a 3 second rule in an effort to get AFCs past the "stage fright" period of cold sell approaches. This has it's merits, but once you're comfortable enough in meeting new people on the fly (do DJ Bootcamp) you have to develop an ability to read your target and this takes a bit of calculation initially.
For example, lets assume you're going to a bar or club to sarge. Before you even set foot in the establishment make some mental notes - What kind of woman goes to a place like this? What day of the week is it? Are you on vacation at a resort? What kind of place is this (a goth bar, a martini bar or Jimmy Buffet's Margaritaville all require shifts in approach)? What's the typical age range for the place?
Once you've established a good understanding of environment lets go further and assume you find an attractive target. First and most important, has this girl given you IOIs? Eye contact, hair twirling, leg shifting? If so you'll have to adjust game for that, if not, you'll have to adjust to catch her attention. Next, read her appearance - what is she wearing? Is she in business casual (just got off of work), or made up in a short red dress (obviously looking)? Is she wearing a wedding ring? Estimate her age and education level (using "chick crack" works wonders for the less educated). Women are by nature attention seeking; virtually everything about a woman is written in her appearance, particularly so while deliberately presenting herself in a covertly competitive social situation (i.e. a club where people go to meet other people).
After an initial read, then look around your target and read the social setting and immediate environment. Is she part of a group of girls (most likely)? What do they look like? Are they feeding her, or feeding off of her attention? Is it a bachelorette party? Are there guys circling them or in their party? Any AMOG potentials? Do any of them look related (obvious twins or a family resemblance)?
All of this will help you apply your game more directly. C&F, neg hits, shut-outs, takeaways, openers, all of that can be more refined and more purposed if you take the time to observe your target and then make some calculated assumptions. A lot of people will argue that it takes too much effort to be that analytical, but after a while you'll become sensitive to this reading ability and it will become second nature.