Like people and gourds, antennas come in all shapes and sizes. But unlike people, not all antennas are created equal. (The jury is still out on gourds.)
The major cellular providers in the U.S. may use two separate frequency ranges for PCS data service--the 800 MHz band and the 1900 MHz band. Some antennas are designed and sold as "dual band," with reception at both bands. But unless you plan to move your antenna to other locations, it is likely that in your area, your cellular provider uses just one of these bands for broadband. Furthermore, it is likely that band is 1900 MHz, by far the most common band for EVDO signals.
Although most people in most places will need only 1900 MHz reception, you can verify this a few ways. One, call your provider. They may or may not be helpful. Two, access the "field test" screen of either your cell phone or your cellular modem. Instructions vary widely by model. Users of Verizon Access Manager software can press CTRL-D and enter the password diagram, then click "Field Test." A window will open with lots of statistics about your connection. It does not specifically show which band you are on, but you can usually infer this from the "Channel" reading. Looking at charts for PCS channels used in 800MHz and 1900MHz ranges, you can probably find your channel and thus deduce your frequency band.
Besides frequency, antenna reception is defined as either "omni" or "directional." An omni antenna can pick up signal coming from any direction and therefore doesn't require precise aim. Directional antennas, obviously, do need to be aimed at the signal tower, but offer the advantage of potentially offering much larger gains.
Omni antennas are most useful for people who can receive a usable signal and want a small boost up to +5 or so dBm. Popular omni antennas include "magnetic mount" models with bases that are self-standing on desktops or car roofs, or the venerable "Wilson Trucker" all-weather omni antenna, which must be clamped onto a host surface for support.
Beyond this level of gain, it makes more sense to look at directional antennas. The "yagi" [below] is a very common directional antenna that comes in a variety of sizes. It should be aimed toward the tower for best performance. A 16-inch yagi properly aimed can offer a boost of as much as +15 dBi, which represents as much as a 500% increase in signal power.
In the real world, a variety of factors can reduce the performance of the yagi, or any directional antenna--or any antenna at all, for that matter. Building construction, hills, and trees will all reduce signal strength from the tower.
You can't do much about hills and trees, but you can avoid the negative effect of walls and siding by moving your antenna outdoors. Generally speaking, all antennas will return better results outside. But—and this is a big ‘but’—the key is not to use so much cable that its losses negate your gains.
In a perfect world, your antenna should be outdoors, but as close to your cellular modem as possible, to keep the cable short. But, the world isn't perfect, and "as short as possible" could be 5 feet for one person and 50 feet for another. (Cellular modems aren't weatherproof, so these must stay indoors, even if that means your attic.)
Remember that a USB cellular modem can be plugged into a USB extension cable. If necessary, you are better off with 10-15 feet of USB cable rather than 10-15 extra feet of antenna cable, because the USB cable won't incur performance loss (assuming it stays within USB cable length specifications).
A Wi-Fi find
One of the best performing antennas for PCS cellular actually isn't a cellular antenna at all--it is a Wi-Fi grid antenna designed for 2400 MHz reception. Sold online by HyperLink Technologies, the HG2424G 24 dBi High Performance Die Cast Reflector Grid actually loses only a small amount of gain at the EVDO 1900 MHz frequency. Although there are similar grid antennas for Wi-Fi with similar specs sold by other vendors, I can vouch only for this model from personal experience. Don't assume that all antennas rated for 2.4 GHz Wi-Fi will also work for PCS—it is possible that competing products would perform similarly well--or not. Anecdotal reports on the ‘Net, however, do report success with other 2.4 GHz antennas.
I purchased the HG2424G after failing to get a usable cellular signal with other antennas. My location is surrounded by trees over 70 feet tall and blocked by a 200 foot hill, in addition to being a couple of miles outside Verizon's official coverage area for service.
By aiming this parabolic grid antenna into the forest, but precisely toward the tower I determined to be closest to my location, I managed to boost an unusable signal to an average of -92 dBm. In real world performance, this has translated to download speeds of about 1.2 mbps and upload speeds just under 300 kbps.