Here is what I'm doing with mobile HF antennas, Fall 2011.
I enjoy working some DX while I'm going to and from work. I'm in the car about 35 minutes each way.
I park in a garage at work, one with only 9' clearance. The HF antenna mount on my bumper is 20" over the ground. So the biggest antenna I can fit on my car and still get into the garage is just a bit over 7feet in length. Since last spring I've been fiddling with my mobile antennas, trying for more efficiency (and less loss).
Obviously, I want the longest antenna I can fit in the garage, to maximize efficiency. If you are interested in HF mobile, you probably know K0BG's website--a fantastic reference for HF/mobile newbies. Reading Alan's article about capacity hats convinced me to try using cap hats on my whips, to lengthen the antenna electrically and thus make it more efficient.
Using a capacity hat also lets me use less inductance in the antenna's coil (the resonator part), giving me less loss there as well.
By making the antenna appear longer electrically, the cap hats lower the resonant frequency. Or, for my own situation, letting me have a longer "virtual antenna" with the same physical length as the non-cap-hat version.
I've been surprised at how much the resonant frequency can be changed by adding the cap hat; I tested a new standard RM-12 resonator/whip a few weeks ago. Stock from the box, the RM-12 and its whip would tune up as low as about 21MHz on my test setup. Adding a cap hat on the same length whip brought it down to mid-15MHz range.
The setup I'm using right now is the best I have ever had, mobile. A bumper mount, a 54" aluminum mast from DX Engineering, a spring: These stay in place. On the top of the mast is a Hustler resonator and whip, which are changed for each band. To change bands, I unscrew the resonator/whip combination that's on the antenna and screw on the one for the band I want; no further tuning or adjustment has been necessary.
For 20meters, I use an RM-15 Hustler resonator with a whip and one of those homebrew cap hats; the total length of resonator and whip is 31". For 17meters, I'm using an RM-15 with a short whip and cap hat, total length around 16". I'm going to change that to an RM-12 to get a little more physical length on 17meters.
For 15, I'm using an RM-12 with a longer whip and a big round corona ball (it's the whip Hustler sells for their high-power 40m resonator.) This is about 31" total. I set up all of these using one of those MFJ antenna analyzers--those things are GREAT for the antenna
The photo here is just the resonator and whip with a homebrewed capacity hat on the top. I make these out of brass rod, two or three 12" pieces crossed in the center, with brass wire around the outer edge. The brass rod and wire cost me about $2 at my friendly local hardware store.
I solder the crossed rods onto the end of the whip, get them arranged, and then run the circle (or two) around the perimeter, soldering with one of those little butane torches.
These antennas are working great for me. By my best guess, the setup I'm using now is a couple of S-units stronger than a standard Hamstick (which I used for years.) With the Hamsticks, I needed to hear someone at least S5 in strength to have a good chance at working him. With the current setup, I'm working people who are only S3. (For comparison, I tried one of those 36" mini-HF antennas on a roof mount, but I needed to hear the other guy at S7 to S9 to be able to work him.) "How weak a station I can work" is the best measuring tool I've found for mobile antennas.