Here's a list of some easy and affordable door treatments that I've found to greatly improve the clarity, as well as the total bass output and low end extension of your mid/bass. If you're using high quality drivers in the doors, proper treatment and install is a must.
First thing's first. Make sure to sound deaden your door. This will make the largest difference in reducing annoying rattles. Make sure to do both the inner panel and the outer panel. Use a good 2 or 3 layers. A heat gun or even a hair dryer can be used to soften the deadener up for making it more moldable and easier to apply. Find a good asphalt based mat that is cheap, easy to work with, sticks well under room temperature, and doesn't fall off in extreme heat. I use and recommend Raamat which you can find here: http://www.raamaudio.com/
60 sqft of deadener should be more than enough for 2 doors.
As for liquid sound deadeners, I don't use them for a simple reason. They take forever to apply! You would need to clean your door, then apply a single coat. Allow it to dry, then apply another coat. With thick coats and bad weather, it can sometimes take up to a full day to dry between layers. I'd save the liquid deadener for hard to reach places, or for areas where mat doesn't stick easily such as the roof of the cabin or trunk. In my experience, the effectiveness is about the same as a decently thick asphalt based deadener.
Also, you can sometimes reduce annoying door mechanism rattles by applying a bit of thick grease to the part.
The next thing I like to do is seal up any large holes in the door panel. Doing this very noticeably increases the bass output. I like to use plexiglass since it's somewhat cheap and weatherproof... and also looks nice. It's also much sturdier than trying to stretch sound deadener over a large hole. Cut out a piece that fits your hole, use a bit of silicon, liquid nails, or other thick adhesive/sealant and then slide it in. These panels can be easily removed with a screwdriver worked into the edges if maintenance on the door is needed.
Next, I glue a large sheet of egg carton foam behind the speaker location. These do hold water, so you may want to treat it to avoid mold growth. It won't rust your door however, since the foam sits on top of the sound deadener. If you live in a more humid climate, you could use a "Deflex" pad which is sold here: http://www.partsexpress.com/pe/showd...number=268-242
You should notice a slight improvement to your midrange. For me, the egg carton foam was a bit more effective than the Deflex pad.
Last but not least, buy ~2lbs of non-hardening modelling clay and a small sheet of 3/4" or 1/2" mdf. Cut a ring or baffle for your speaker to sit on. Place about 5mm tall height of clay on both sides of the ring. Now mount the speaker to one side of the ring, and the ring to your door. Decoupling the speaker from the actual door itself will further reduce vibrations, and clean up your midrange and bass. As an added touch, I like to add a bit more clay around the baffle in order to add weight to the area and further dampen any vibrations.
3/4" mdf baffle with non-hardening modelling clay atop.
Seas Excel w18 with non-hardening modelling clay around the baffle.
Notice the plexiglass + liquid nails which was used to cover the hole in the door panel. Also, notice the 3 layers of deadener on the outside door panel through the glass.
Sheet of egg carton foam behind the speaker.
Deflex pad behind the speaker.
Shot of my trunk lid with asphalt based sound deadener applied.
Shot of my trunk with about a 4mm layer of liquid deadener applied.