Your best bet for learning how to play VP is to download one of the software tutors. VPfW, Winpoker, FVP, Optimum VP, Vipoker, etc. You can use an internet search engine to find where to get them.
Positive games are ones that return more than 100% of your bet over time using perfect play. Easier said than done. The best DDB game is called 10-6 DDB because it returns 10 for a full house and 6 for a flush. These are the only pay tables lines that are typically changed for DDB. I suspect the games you've been playing return less.
Perfect play is a hold for any given hand that will return the most over time. In another thread it was mentioned that a dealt king high straight flush, when held, returns 50-1 (250 for a 5 coin bet). That is the expected return of holding all five cards. The expected return of holding just the 4RF was around 19-1. Again, that is the average over many, many holds of this particular hand.
If one examines ALL possible deals (there are 2598960) and reviews ALL possible draws (there are 32 possible draws for each dealt hand), you can determine the best hold for every hand. Perfect play is always selecting that best hold. A games expected return (ER) is based on perfect play for every hand and is often called max-ER.
This is not as difficult as it might seem at first. The rules for any game can generally be kept on a couple of index cards. Naturally, the strategies are different for every game because the payouts for the various paylines are different. In the example above, if the straight flush returned only 15-1 then holding the 4RF would have been the "perfect play".
Since no one plays absolutely perfectly they can expect a small reduction for their own personal ER. I use reductions of .05-.25 depending on the difficulty of the strategy. 10-6 DDB returns 100.067% with perfect play over many, many hands. The game is not too difficult so you might want to subtract .1% yielding a personal ER for this game of 99.967%.
One question typically asked next is "how many hands does it take before I see the expected return?". Once again the answer is different for every game. For games that have large, infrequent jackpots (like DDB) it will take longer. For games where more money is returned in the more frequent hands (like Jacks or Better), it takes less time. Essentially, as you play more and more hands you will be approaching your personal ER for whatever game you play. Don't be fooled by those claiming they have a better short term approach. Your results over time will approach your personal ER. Using anything but perfect play will reduce your personal ER.
In another thread (the discussion of "poor play") you saw examples of imperfect play. Every time someone makes these types of plays they are reducing their personal ER. Since the vast majority of gamblers aren't aware of perfect play strategies or don't want to spend the time to learn them, the casinos can put in what we call positive games (>100% max-ER) and still make a reasonable profit.
Please feel free to ask questions as this can be a very daunting subject to a newcomer. Also, you might want to look over some of the information provided at VPfree:
http://members.cox.net/vpfree/FAQ.htm#Dog
Other links:
http://members.cox.net/vpfree/PayV.htm
http://members.cox.net/vpfree/Links_C.htm