Hey Kag, glad to see you decided to post here

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There are a couple key factors to consider when deciding where to bluff:
First off (and quite possibly most important), there's the skill level of your opponent. Simply put, beginning players are very hard to bluff - they'll call anything with top pair no kicker. So, simply put...don't bother bluffing them.
Next, make sure your bluff tells a story, you really do need to sell it or people we see right through it. If you've been playing aggressively, and check a flop of KT7, then bet the turn hard (K), no one's going to believe you have the trips! Now, let's say you hold QT on a board of JK78, with 2 clubs out there. You call a small-medium sized bet, but miss your open ended draw. You do notice, however, that a third club hit the board. If you believe your opponent doesn't have the flush, you may very well be able to bluff him out of the pot, as a strong bet suggests that you hit your flush (just be sure that your opponent doesn't have it, though, as this could put you in a world of hurt).
Finally, there's my favorite bluff, the check-raise bluff (NOTE: THIS PLAY SHOULD BE USED SPARINGLY, IF AT ALL). This works best when you are in the blind, and a small pair hits the flop, with another low card (e.g., 664). Let's say you have 57, simply to give you some outs if you are called. Check-raising has a number of benefits here, as it portrays you holding one of the 6s. Now, if you happen to run into someone who does hold the 6, you're in a lot of trouble...but that's why you have to be very careful using this play. There are other positions where this may work, but for the most part you just have to get a feel for the table before trying anything too tricky. Again, notice that I told a story here - if it's not believeable I'm calling you the entire way down.