Through Supplements
If you are looking for a list of supplements to take to boost testosterone, we recommend you look at our increasing testosterone supplement stack
Through Lifestyle
A dietary Zinc deficiency can hinder testosterone production, and thus in order to get high testosterone levels it would be prudent to avoid dietary reductions. A good zinc status can be achieved through either supplementation (such as a ZMA formulation) or through high meat and shellfish consumption.
Similar to Zinc, Vitamin D can also regulate testosterone levels. However, 'deficiency' is a bit different.
The RDA/DRI for Vitamin D tends to be around 400-800 IU (International Units) per day, depending on country of origin. This is a good level to aim for from food, but it may still be in the range of 'subclinical deficiency'
Year-round supplementation of 3000 IU Vitamin D can increase testosterone levels by maintaining serum vitamin D above 30nmol/l[1] and increases a bit further around 50nmol/l.[2] This is primarily due to vitamin D being able to regulate the aromatase enzyme.[3]
If you live far away from the equator or otherwise stay indoors most of the day, supplemental Vitamin D would be needed (as dietary sources give relatively low amounts)
Avoid deficiencies in important nutrients. Lifestyle is the first frontier for deficiency avoidance, but supplements can act as band-aids in the case if one cannot implement lifestyle changes
Other Techniques
At least one study has noted that men have higher testosterone levels when percieving the scent of an ovulating woman, relative to non-ovulating.[4] This testosterone increase from olfaction appears to be consistent across measured species.[5][6] Increases in testosterone in response to scent are relatively non-existent in the above study[4] but a decrease was seen with non-ovulating scents; so it appears that ovulation attenuates a decrease that is caused by another factor. The researchers hypothesized these fluctuations were in response to perceived fertility. (It should be noted that no females were present in these studies on scent, as to control for female interaction)
Watching sexually arousing films (porn), or just being sexually aroused, can increase testosterone secretion in men[7][8] watching 4 minutes of erotica can also increase subsequent performance on a 3 rep squat[9].
The increase when sexually aroused is minor and transient, with 25.2+/-6.3nmol/l in the sexually aroused group and 22.2+/-5.6 nmol/l in control. The arousal does not need to be great, as testosterone increases are seen even after social interaction with attractive females.[10][11] After a 5 minute conversation with an attractive woman, male testosterone levels can increase up to 30% from baseline (relative to an increase of 13% for conversing with males).[11] This degree of testosterone increase is unlikely to exert any anabolic effect on muscle tissue, but can contribute to neurology.
Interestingly, the level of interest females show does not seem to matter in regards to testosterone increases (as measured by flirtatious v. aloof confederates) nor does the female's perception of how the male may have reacted to their advances.[10][11]
It should be noted that although the testosterone increase from sexual perception diminishes shortly after sexual appetite is diminished, it is not reduced; Ejaculation does not adversely affect testosterone levels.
The color red has also been implicated in increased percieved attraction, greater parameters of social success, and confidence when worn by or associated with a man.[12] Although the color red does not directly increase testosterone, it increases correlates which may have an affect.
Be around attractive women and display power, perhaps have sex a lot. Just don't make the women sad or they'll sap your testoterone (crying females reduce testosterone[13]) Red would help as well; and none of what was just said is miraculously effective for muscle mass growth.