Summarize:

While you can't control your genetics, you can use the power of illusion to make your hair appear thicker. Ask your hairdresser for the best cut and style to create thickness and volume in your hair. Get your hair trimmed every 6-8 weeks to keep it looking fresh and full.  Add layers to create fullness, especially if your hair is very straight. If you are feeling daring, consider getting bangs. The contrast between your bangs and the rest of your hair will help accentuate length. Highlights add dimension to your hair, making it appear fuller. However, bleaching your hair extensively can cause breakage, so stay within your natural color range, or ask your stylist to use semi- or demi-permanent dyes. Hair extensions involve temporarily attaching either real or synthetic hair to your head. They can be held in place using thread or glue, or they can clip into place. Extensions can be a great way to add length, volume, and color to your hair. However, if your hair is already thin or damaged, extensions might not be your best option, as they can cause more damage.  If you have glue-in or sew-in extensions, they must be replaced every 3 months as your hair grows out. These extensions range in price from $100-$1,000 per set. Clip-in extensions should be taken out at the end of each day. They're much cheaper, but they also look less realistic.
Get a layered haircut so your hair will appear thicker. Add in a few highlights to play up your hair's volume. Get hair extensions to add even more volume.