For a sustained boost testosterone, others have already touched on the main factors: lifting heavy / exercise and diet. Here's a blog entry by a guy who claims to have doubled his, with more detail and other suggestions pertaining to sleep, stress, etc.:
Art of Manliness

Most natural supplements like tribulus *might* give you a short term boost, but to the extent there is any benefit, it's going to wane quickly over time and you need to cycle off. At that point, if you haven't put in the work to sustain or increase it through activity & diet, you'll return to whatever your baseline was before.

Aside from that, the only thing I don't think anyone has mentioned yet is an emphasis on activities that require quick power generation. If you're not a city dweller where toting an axe around will arouse suspicion, get a cheap axe, go into the woods or backyard and chop wood. I'm serious. Google, "chopping wood increases testosterone". You will start feeling a sense of vitality and well-being if you do this for 30 minutes or so.

Undoubtedly, chopping wood being beneficial pertains to the generation of power while swinging the axe. To this end, other activities that generate explosive power like working on a heavy bag will be helpful. But, personally, I think there's something unique about chopping wood because of how hard it works the hands. With a good swing, the moment of impact shocks your hands and you have to strain to control the axe, which really works your grip... and there are studies that also correlate hand strength to testosterone in men. So, there's that.

If you have a large hammer or mallet and want to beat on a pile of earth, I imagine it would work similarly well.

It shouldn't need to be said, but-- of course-- you should take common sense safety precautions when dealing with anything which might hurt you and if you've never chopped wood, swing lightly at first and slowly build up the power of your swing so you don't injure yourself.

If doing the above is not at all practical, be creative in researching / finding explosive activities with an emphasis on grip strength.