There are many ways of collecting stag beetles. Here are some of the most common methods.
 

Street Lamps
About two-thirds of the stag beetle species in Taiwan are attracted to lights at night. Why insects are attracted to lights still isn't very clear. Most explanations suggest that bright lights, which don't exist before man, confuse flying insects as to which direction they're traveling. Nights with a bright moon are not good for this type of collecting, which suggests that insects might use the moon for navigation. Anyhow, this is often the easiest method of collecting a variety of stag beetles. The best nights are dark, windless, and dry or misty nights. Foggy, but not rainy, nights are the best because the light is refracted off water particles in the air, creating an illuminated area around lamps. After dark, look for street lamps that are near forested areas. The best lamps are those standing apart from any other lamp or are brighter than all the rest in the area. White lamps are far more attractive than yellow ones. If you are lucky, you may see beetles flying around the light. If so, use a net to catch them, or wait until they land somewhere. Use a flashlight to scrutinize everywhere that is illuminated by the light, including the ground, trees, walls, or anywhere else where a beetle might land. (Imagine that you are a beetle circling around the lamp. What is the first thing you will hit?) Often the beetle may be lying upside down on the ground, hiding in the grass, or it might be clinging to nearby tree trunks or overhanging vegetation. Careful searches often reveal pleasant surprises. Here's a picture of me with all the stag beetles I caught in one night under no more than a handful of street lamps. Most of them were knocked down from the twigs of nearby trees.

Your Own Light
If street lamps attract a lot of beetles, than what about setting up your own light? The traditional method is to set up a reflective white cloth vertically using string. Hang or set up a black light and/or mercury vapor lamp in front of the cloth and use a portable generator or battery as the source of electricity. This must be set up before dark, and the lamp should be turned on right before it gets dark. The tricky part is where the cloth and light should be set up. To catch the most insects, look for an open area overlooking a large patch of forested hillside. Setting up the lamp inside the forest will bring in different insects. Of course, no street lamps or other sources of light should be visible from the site you choose to set up your own light. Choosing the location in which to set up requires experience and trial and error. When conditions are perfect, this method often brings in large numbers of beetles and many rare species.
 

Trees
Nearly all stag beetles feed on tree sap, so the best place to search for them is probably their food source. Most stag beetles are rather specific in the tree species that they feed on. Some species like citrus and are easily found in citrus orchards. Other trees that attract a variety of species with their sap include trees of the genus Cyclobalanopsis which are common in mid and low elevation forests. Not all trees will have beetles on them though. Stag beetles often choose to congregate on the trees that are already weak from disease or other insects.

Rotting Fruit
One way to attract stag beetles is to bait them with fermenting fruit. Pineapple, banana, and other fruits with a strong odor work best. Leave these fruit in the forest for a couple of days and let them spoil. Every once in a while, check to see if they've attracted any stag beetles. Be sure to look inside the fruit as well as on the surrounding ground.

Rotting Wood
This method requires both experience and some luck. Stag beetle larvae feed on rotting wood, both fallen and standing, and the adults often hide in wood during the daytime. Turning over a rotting piece of wood or digging through pieces of a rotting trunk may reveal some adult stag beetles. Inside the wood, one should expect to find more larvae than adults. Do not discard the larvae! Either take them home with the piece of wood and try to rear them into adults (see Breeding and Rearing Stag Beetles) or return the piece of wood into its original condition so the larvae can continue to feed. Sometimes you might encounter adults that have shed their pupa skin but haven't emerged from the wood yet. Some species will stay dormant in this chamber for many months before becoming active. This is one method of finding species of stag beetles which are not attracted to lights. The few species that live through the winter as adults may also be found like this when they're not active.

Roads and Drainage Ditches
Some species of stag beetles are not attracted to lights, but they may fall into the drainage ditches along the roads as they wander about the forest. Neolucanus swinhoei and Neolucanus doro are often found crawling along roads where they frequently get run over by traffic. Dorcus clypeatus and various other species can also be found trapped in drainage ditches.

In Flight
A small number of stag beetle species have an interesting behavior of flying above grasslands for no apparent reason. Lucanus datunensis and Neolucanus swinhoei often display this behavior on the grassy peaks in Yangmingshan National Park. Lucanus miwai displays the same behavior in grassy areas on the mountains of central Taiwan. Stag beetles in flight may be knocked down using your hand or a net. Pseudorhaetus concolor is also an active flyer above the forest canopy but is difficult to catch.