Use of an external antenna on your MURS Alert Base will improve your range. Signals will be stronger and your communications range will be better. If you use external antennas on both ends, then your improvement is even greater.
A base antenna needs to be securely mounted to your home or structure. In addition, coax cable connects the antenna to the radio. A lightning arrestor and proper earth grounding should also be included if the antenna is to be connected 24/7. It is also very important to chose a low loss, good quality coax cable or else it will negate any improvements you make with a better antenna. Elevating the antenna above ground in addition to the better antenna will make a significant improvement in your range.
Base antennas come in omni-directional ground planes and directional Yagi antennas. If all your operations are in one direction, then a directional antenna will focus all your radio energy into that direction yielding additional gain. An omni-directional ground plane will transmit and receive equally in all direction and is usually the most commonly used antenna.
Avoid using cheap coax cable. Typical coax obtained from places like Radio Shack is often poorly constructed. Getting the signal from the antenna to the radio is very important and coax you should not scrimped on it. Low loss coax such as Belden 9913, RG-213, and LMR-series coax is suitable in lengths up to 100 feet. The RF Connection is a good source for coax and they can create customized lengths and connectors for you. The MURS Alert Base uses a standard BNC connector to attach the antenna. Centerfire Antenna offers a coax assembly all ready to go with a BNC on one end (for the MURS Base) and a PL-259 on the antenna end.
You should seriously consider lightning protection for your antenna as well, especially if you wish to keep your radio connected to the external antenna all the time. Simply running a ground wire to earth is not always acceptable protection. See this site for information about protection devices and proper lightning protection. You should always disconnect your antenna from the radio during the threat of lightning.
In all cases, using a better antenna and elevating your antenna will result in improved communications range. While you still won't be able to transmit through a mountain, you will find that your signals will be stronger in places that they were weak before.
There are lots of online resources available. Anything that is valid for 2 meter amateur radio (ham) antennas will work for MURS. Many 2 meter ham antennas will work if shortened slightly. Here is a large list of resources, that include antenna theory, building your own, etc., that will get your started - http://www.ac6v.com/antprojects.htm