First of all, you need a great crust. I have a great recipe for an easy crust that comes out perfect every time. Read my article Perfect Pie Crust Made Easy.
I always use a different recipe for this pie. This is one I found on Cooks.com and made a few adjustments.
1 1/4 to 1 1/2 c. sugar
3 oz. package of quick-cooking tapioca
1/4 tsp. salt
1/4 tsp. ground nutmeg
2 c. rhubarb, cut into 1 inch cubes
4 c. fresh strawberries, sliced
1 Tbsp. butter
Mix the first four ingredients together (sugar, tapioca, salt, and nutmeg). I like to grate my own nutmeg. It has a fresher flavor.
This recipe uses tapioca as the thickening agent. I have used flour before and many people use dissolved cornstarch, but tapioca seems to have a good amount of thickening power without leaving the berries dull or cloudy.
Toss the prepared vegetable and berries in the sugar mixture. Make sure it is evenly coated, let rest for the time it takes you to get the crusts rolled out and prepared.
Place fruit mixture in the bottom crust, dot with butter, and cover with the top crust. Seal the pie and cut a vent for steam release. Often I will use a cookie cutter in the shape of a heart or star to make a nice variation of a crust vent.
Bake at 375 degrees for 45 minutes, or a little more depending on your oven. Remember to protect the edge of the crust with a crust saving ring or a bit of foil so it doesn't cook faster than the pie. The pie is done when filling is bubbling and crust is golden.