Neat and even top stitching (or topstitching if you prefer) makes a huge difference in how your finished project looks and lasts.
Here are some ideas for improving your top stitching:
1. Lengthen your stitch. Often when it is time to top stitch you are going to have multiple layers of fabric to sew through, so you are going to need a longer stitch. If your stitches are too small they won't look good, they can start to distort the fabric and if they are really little they can make the fabric prone to ripping.
2. Slow down. Unless you are using a machine designed for industrial production, your machine is probably not made for speed. Slow down and try to sew at a gentle steady pace. Give it a try and see if you notice an improvement in your stitches.
3. Use the hand wheel. Anytime the fabric is extra thick--like at the side seams--take your foot off the pedal and use the hand wheel to drive the needle. Gently pulling the fabric while you turn the hand wheel can help too. The feed dogs have less momentum without the motor so pulling (gently) can help keep your stitches even.
4. Use the edge of the presser foot as the allowance. It is very stable, accurate and easy to follow guide. Using the edge of the presser foot as my guide has by far made the biggest impact on the quality of my sewing--for both straight stitching and top stitching.
5. Try a top stitching needle. There are special needles available for top stitching. They have a large eye and a very sharp point and are helpful for top stitching on very thick layers of fabric. I find the universal needle works pretty well as long as I have a nice long stitch.
6. Top stitch from the lining side of the bag. Even if my bag will go around the machine arm, I still top stitch from the lining side (see photo at top). When you open your bag, the first place your eye goes is to the back wall of of the lining and often this is where the most erratic and wobbly stitching ends up! Then if your thread is not the same color as the exterior fabric, you end up with an even more visible mess. By top stitching from the lining side you can avoid these problems.