Strategies for media buying?

What are some of your best strategies & hacks for buying traffic?
Display, email, CPC, CPA, etc.

One hack I use when paying on a CPC basis is to discourage clicks.

Your ad is going to be shown to a lot of people. Most of them aren't going to convert in the end. When you're a startup with a limited budget you need to get the most out of, preventing people who won't convert from even clicking on the ad makes your ads much more effective.

Things like listing the price of the product / service in the ad copy helps prevent people without the ability to purchase from clicking. Mentioning what your product doesn't do (implicitly) helps prevent people looking for a different kind of solution. (e.g. MailChimp's ads could say "The best opt-in email marketing solution for small businesses.)

When you're out of startup stage and have more of a budget, there's benefit to be gained from educating the market / getting people interested who aren't qualified themselves but may know people who are. But while you're a startup, the more you can help ad viewers disqualify themselves (without creating ads that aren't a turn-off in general), the better end-goal conversion you're going to get for the budget you spend.

While not a hack per se, I hope you have retargeting media buys in your mix. AdRoll is the way to go for retargeting. It's not unheard of to get 10x+ ROI when retargeting.

Also, spending time to create your target customer's profile in detail is worth the effort. You'll have an exact blueprint with which to segment and target your ads.

When starting out with Ppc with a small budget target keywords with intent that shows higher order values such as - bulk , commercial, wholesale, etc when applicable and stay away from broad match keywords.