I don't use space planes per-se but I lots of experience with SSTO's that use jets for atmospheric propulsion so these tips should apply to spaceplanes too.
First off, use
MechJeb and (optionally)
Engineer. Mechjeb can control your heading and pitch much better than the current stock SAS. Engineer will let you watch your orbital numbers in real time. Mechjeb can also control your throttle much more finely than you can and can prevent flame outs all the way out of atmosphere.
Here is what my set up looks like when launching.
For the design, like Urist McScoopbeard said, make it as light and efficient as possible. Especially the space stage, you souldn't need very much engine at all once you're up there. Since you're going to be spending most of your in-atmosphere time at 20km-40km only use turbojets since they're more efficient at that altitude and don't require cooling. One common trick for air intakes is to put cubic octagonal struts on the side of your craft and attach Ram intakes to them or to stack cubic octagonal struts inside each other and attach circular intakes to them. To get a good idea what's possible go though
the MachingBird challenge thread on the KSP forums. Some of the designs are just
obscene.
When flying you'll want to get up to 10km as quickly as possible then slowly level off until your apoapsis is 20km. Engineer is great for watching your apoapsis while doing this part. From there on you'll want to keep the angle at about 5 degrees. Then it works mostly like bringing a standard rocket to orbit. As you gain speed your apoapsis and periapsis will raise until eventually your periapsis will be just below you and your apoapsis will shoot up. The goal is to use as much of your jet stage's fuel and as little of your space stage's fuel as possible before exiting the atmosphere just below 70km. When you exit the atmosphere completely your periapsis should be around 40km-60km and your apoapsis should be somewhere above 100km. This should make it so your space stage only needs 100dV-200dV to get into a stable orbit.
EDIT: I have shift+space set to start playing Winamp and pressed it while trying to launch.
Guess what song it was on.