You're probably well aware but most of those places will try to give you free credit to "start off", keeping the crack dealer reference going. It's never enough to really figure things out but it's still a good deal to take.

Know your product ! Figure out if you want traffic or subscription (are you willing to take a signup loss for long term gain ?) or leads and how qualified.

When you use adwords, they keep offering you to talk to a google representative every couple of month, to review your account and make suggestions. I found those a massive waste of my time when I had a campaign going well, but when I had trouble launching something new they were often a very decent (and free !) help. Make sure to use that and ask any questions you may have then.

Like I said they want to extract as much $ from you as they can, but they clearly know they need you to make money from it too. It's a love / hate relationship, but it works for me.

Best of luck on your venture