I take it that for a tart shell you mean shortcrust pastry (otherwise known as pie dough or 3-2-1 pastry)?
I will give you a rundown on how I personally make my pastry and I'm sure others will do the same (note: this is for an unsweetened pastry as that means I can adapt how sweet it is according to how sweet the dessert is, or leave it unsweetened for savoury pies/tarts and quiches). I follow the method taught by the Culinary Institute of America (the good CIA :P).
2 cups plain flour
150g chilled unsalted butter, cut into 1/2 inch cubes (5 1/2 ounces)
ice cold water
In a large bowl combine your flour and the butter. Work it with your fingertips (like your trying to squish flour into the chunks of butter). Continue doing this until:
a) some of the mixture looks like breadcrumbs but there are still thin flakes of butter (about the size of a dime, I think that would be the right American coin size). This will yield a crust that will be quite flaky.
b) all of the mixture resembles fairly coarse breadcrumbs. This will produce a much less flaky crust, and holds its shape better when you cut it because of it.
You could do the above steps in a food processor fitted with the blade attachment but it is hard to end up with a flaky dough.
Now sprinkle the flour and butter mixture with a little of the iced cold water. Work the liquid through the dough by lifting and sifting it with your fingers. Add water (in small amounts) until the mixture is lightly moistened (note: if you think its too wet, it probably is).
On a clean work surface empty out the contents of the bowl. Form into a fairly thin (bit less than a half inch thick) circular disk. Wrap in plastic wrap and place in the fridge to chill. (It's a good idea now to place a piece of marble or granite, if you have it, in the fridge along with your rolling pin.)
After about 30-60 minutes (you are waiting for the butter to firm up again, you cannot let it melt under any circumstances) whip your dough out and begin to roll it out on a lightly floured surface with a lightly floured rolling pin (best way to do this is to not let the rolling pin go over the edges of the pastry, stop it before hand, give it a quarter turn and roll again, turn again, roll again etc etc). Roll it out until it is the size you want (always try to roll it a little larger than the tin, most pastry recipes allow for a little extra to account for this still keeping the thickness you want).
Now place in your greased tart tin (I find the easiest way to do this is to lightly flour the top of the dough, fold in half to create a semi-circle, lightly flour the top again and fold in half again to make a quarter circle. Place the point of the quarter circle in the middle of the tart tin and fold the pastry out. Now press the pastry into the tin (using scraps of pastry if possible) and tidy up the edges.
You must bake blind if you are going to fill the tart tin with ingredients which will not undergo further cooking, or if they are only going to be baked for a short while (not enough for the pastry to cook).
Preheat an oven to around 350'F. Line your tart shell with al-foil and place pie weights/dried beans/rice in it up the rim. Place your shell in the oven and bake for around 12-15 minutes or until it is lightly golden (or a little bit longer if its not going to undergo any more cooking).
Phew, that was an epic. What's the bet that after I hit reply 50 other people have responded before me even though nobody had before I started typing!