Low voltage systems basically involve a main power box that converts regular house current to the low voltage current used in these systems. There is a common misconception that the level of power at the beginning of the line will be the same at the end of the line. So for a 300-watt system it's easy to be convinced that you can have as many lights as desired and never fall below 300 watts. This isn't quite true.
Voltage drops are common problem with low voltage systems due to resistance. The more fixtures on a line, the more resistance. The current travels down the wire line and has to go through each fixture, hitting more resistance, which aggregates. Thus the lights at the end of the line can be much dimmer than those closest to the power source. It's better to not max out the lights that a line is capable of, and instead use fewer well-placed lights to reduce resistance.
Accidental Cuts in the Wiring
A common problem with installation or in after-care, cuts in the wire can be irritating and hard to find. A line laceration is actually most common with landscaping after the system in installed. It doesn't take much effort for a pair of long snippers, branch trimmers or a lawnmower to slice through these cables.
Many homeowners who have low voltage landscape lighting have problems with cut power cables. It's easy to forget over time where shallow buried cables lie. To avoid this, try to plan your cable runs so they might not fall where you will possibly landscape in the future.
Junctions are also prone to causing problems. These are required where, for instance, you set up a t-shape system where a power line has to connect to another wire running 90 degrees (think the side of your garden running along the side of the house). If the junction is not protected or there is too much load, it can melt. The junction box is a cheap little $1 plastic metal bridge with teeth to carry the current. So it's not going to handle a hot load very long. With a bit of moisture or a lot of lights, you will get a meltdown and a short or ground. Then a whole leg of your system will go out.
If your light bulbs are burning out quickly, take a look at the quality of the bulb itself. Typical of many hardware stores, cheaper bulbs come from overseas. Yes, you save a little bit of cash up front, but you lose it constantly having to buy replacements.
Installation Care Saves Money
You can also be your worst enemy to your low voltage bulbs. Human fingers are covered with small amounts of oil. This is the medium by which people leave fingerprints on what they touch. Unfortunately, this same oil on a light bulb can cause hot spots and weakening in the bulb glass. That in turn causes the bulb to fail. Wear gloves that allow you to gently hold the bulb, and you can save yourself a lot of hassle.
Rodents and the Elements
For outside wiring, furry critters can be a challenge. Rodents of all kinds like to gnaw on wiring. Whether it's rats, gophers, squirrels or mice, their taste for chewing on rubber insulation is common and shared. The best way to fend off this problem externally is to bury your main lines so they only come out of the ground where the lights are installed.
Regarding the elements, water will be a problem, but most of your wiring will be insulated well. What can cause damage, however, is the sun. The solar rays will dry out the rubber cover, eventually causing it to crack. Again, if not buried, your wiring will be exposed through cracks and will begin to short.