Flush draws: When to keep going and when to give them up?

[Wahoo73]

I'm now nearly two months into my seemingly never-ending slump. After having a nice four day turnaround last week, my weekend playing in cash games wiped out those gains, despite finishing ITM in a stud tourney and a NLHE tourney. The story was pretty much the same as it has been for the last two months...big starter pairs that don't improve, big pairs that improve to two big pairs but get beat by little trips, OESDs that never finish, and lots of live four-flushes on 4th or 5th street that don't fill.

In doing some in-depth hand analysis, I'm finding it is the latter in particular that are costing me a lot of money. (Particularly frustrating is when I start with a four-card flush with none or only one of my suit visible and by 6th street I still haven't hit and no more than one more of my suit has shown up in other hands...and of course the river is a blank.) Over the last two months, my starting four-flushes have only completed 18% of the time...and of those that have completed, 22% have still lost. These numbers seem abnormal to me. Are they?

So when do you keep drawing to your flush and when do you fold? I realize that there is no "one-size-fits-all" answer to this question, but I'd like to hear some general "rules of thumb" from the forum beyond the basic guideline of "don't play your starting flush draw if cards of your suit are door-cards in three opponents' hands."

P.S. I'm not looking for a math probabilities-type answer. I'm familiar with 7-Card Stud Flush Completion. I'm more interested in tactical advice on playing flush draws on 4th street and beyond.