Darts are a nice way to give bags depth so I use them a lot. Here is the method I've come up with to make sure they are the same size and match up well.
1. Use a fabric pen or chalk to draw your darts.
2. Stick a pin in one side of a dart and out the other coming out right in your dart line on the back side. Then flatten it and pin in place for sewing.
3. After sewing both darts on one side compare them to each other. Are they the same size at the bottom? Are they the same length at the top? If not go back and make adjustments until they are pretty much the same size.
4. After sewing all four darts on your two pieces make sure they all line up pretty nicely. Sometimes on one side the darts are off and lay outside the darts on the other bag side. If you need to make adjustments make sure you do it to both sides at this point. Adjusting just one dart now might leave you with a lopsided bag and it is hard to notice until you are finished with the bag and it's too late to go back! Just sew both darts on the side that needs adjusting a little bit bigger until they all line up.
5. If you are sewing darts in clothing you will want to clip the triangle of fabric off. If you are making a bag leaving the extra fabric in place is fine. Simple fold the top dart one way and the dart on the other side the opposite direction.
6. Align the two sides matching the darts closely but not perfectly--I find it works best if the top dart of the first dart set you will sew across is just slightly in front of the bottom dart. As you sew around to the dart the fabric gets thicker and the presser foot will roll the top dart right up on the lower dart. If you line them up perfectly it seems like the top dart gets pushed off the lower dart and ends up slightly misaligned behind the lower dart when you are finished sewing.