Must Have Techniques

What are the most common techniques you use in your PP and which ones are more requested by clients? I really want to expand my post work to cater more to clients. Brenzier etc

Depends on the subject matter.

But pro photographers use canned filters a lot more than you think, it saves time and thus money in the long run, I'm talking about stuff like:
LR presets - just google about and soon enough you'll have a whole bunch if LR is your thing.
Nik plugins - lots of post effects and tools and all of them are good.
Alien Skin Exposure - some of the color filters have very unique properties when used right, it's my secret weapon for pop in images.
Imagenomic Portraiture - doesn't replace skin retouch, but when used in moderation it's very quick at doing the most tedious thing for me, which is uniformity.

Of course, if you're doing landscapes, it pays to know how to use LAB color space. Actually it pays to know how to use it in general, it solves the occasional headache.

Most requested stuff for me that isn't shooting test charts is B&W and portraits, which sucks, because I don't especially like doing either.

Levels/Curves affect the whole image. OTOH, Adjustment Layers which have a mask built in can be modified to only affect that part of the image that you think needs it. And they're reversible in case you need to tweak them later for a print.