Pressing
The texture of firm and extra-firm tofu can be altered to be more dense and firm by pressing it. When you read recipes involving tofu, you will often see "firm tofu, pressed" under ingredients. Pressing tofu also allows the tofu to absorb sauces and marinates more readily.
For Christmas, I decided to splurge and buy a tofu press, but using the heavy object method works fine too.
To press tofu,
Slice tofu to your desired thickness.
Wrap it in a paper towel so it doesn't get lint on it.
Wrap the tofu in a clean dishtowel and place it on a plate.
Add something heavy on top, like a cast iron skillet or thick books. If you are adding small items on top, you may want to first place a baking sheet or plate on top of the tofu so they have a flat place to rest.
After 20 minutes or so, remove everything and unwrap the tofu. The dishtowel will be wet since it's absorbed the liquid from the tofu and you will notice that the tofu is much more dense.
Freezing and Defrosting
Freezing and defrosting tofu changes its texture even more than pressing does. It actually makes tofu chewy. This texture lends itself to recipes for which you might want a "meatier" texture.
To freeze the tofu, you don't even have to open the package. Just pop the unopened package in the freezer. Freeze for at least 24 hours.
Defrosting takes a while, so plan in advance. Optimally, tofu is defrosted in the refrigerator overnight or one day in advance. However, if you don't have a lot of time, you can remove the tofu from the package and microwave it on the defrost setting for 10 minutes.
Once it's defrosted, squeeze the water out and press it for about 30 minutes to achieve its full chewy potential.
It's normal for defrosted tofu to be slightly yellow in color.
Try this recipe for Microwave Sticky Tofu Ribs.