There are a number of boosters available, the lollipop to remove specific squares is useful to overcome the trickier jelly levels, but I've only ever used the free boosters so can't add much to this. I also deliberately do not play with friends or share on facebook. At the end of each 15 level episode you can complete a number of 'quests' (additional levels or reruns of levels you've already seen) to unlock the next episode.
The difficulty of the levels doesn't necessarily increase as you progress through, a google search will normally give some indication of how many others out there are sharing your pain in not being able to progress through level xx. Some of the 'score xx points' levels will reduce the target if you are consistently failing.
At the end of the day it is a somewhat random game (I think of it as a very small fruit machine), I don't claim to have found all of the rules, there are still occasions when 'stuff happens', but the following might be of some use, beyond the basic 'combining three of a kind'. Although there is a lot written about the game a lot of what is out there is poorly written.
Striped candy- when you combine four of a kind in a row or column. If the final move to build a column is a horizontal move the resulting candy will be horizontally striped and will clear a row when activated. If a row of four is build with a vertical move you'll get a horizontally striped candy which will clear a column. Simplest activation of the striped candy is to combine it in a subsequent three of a kind. Wrapped candy- is created when you build a T shape or L shape (in any orientation). When activated in simple mode it will explode clearing the candies surrounding it, it will then drop (while flashing) and do the same again. Chocolate button- is created when you get five in a row. This candy can't be actively moved, it's simplest activation is to move any other candy into it. This will then eliminate all candies of that colour on the board.
Super combinations Instead of simple activation of the super candies it's possible to combine then to greater effect. Guide here: http://candycrush-cheats.com/special-candy-combinations/ (my chocolate button is their color bomb)
There are five basic flavours of levels- Target score levels – get a minimum score to pass Jelly levels – clear all the jelly to pass Ingredient levels – bring down all the ingredients to pass Timed levels – get a minimum score Candy order levels – fulfill certain candy objectives
If you're playing on a mobile device you can exit out of the level (except for the timed levels) before making a move and not lose a life. This can be useful as the starting layout of the candies is random, so for the more difficult levels you can reset the board until you see a layout you like.