While going subcutaneous (sub-Q) is almost unheard of among old time testosterone injectors and steroid users in general, it's actually the way to go if you're using small, TRT-sized doses (200 mg. or less).
Rather than sticking a large needle in your behind, you stick a small (26 gauge, 3/8" needle) on either side of your abdomen. This method appears to have several advantages:
Going under the skin saves you from jamming yourself in the glutes approximately 500 times every decade, thus saving you from a lot of scarring.
It makes it more feasible to dose yourself twice each week, which allows for a more even and natural testosterone profile.
Since you're injecting two times a week, the testosterone is more smoothly metabolized and might even bring estrogen levels down (and research seems to support this).
The late Dr. Crisler, who helped popularize this method, believed that it gives you more bang for your testosterone buck. He said that 80 mg. of sub-Q testosterone has the effect of a 100 mg of testosterone administered intramuscularly. (Whether Crisler was right about the potency of sub-Q injections isn't known for sure, but it has the ring of truth and it's worth a try.)
You can use the same needle to fill your syringe and administer the dose.
Since there's no danger of puncturing a vein, there's no need to aspirate when you go sub-Q.
Here's Dr. Crisler with an injection demo:
Just swab an area of skin on either side of your belly button. Swab in concentric circles, starting small and working your way out (it physically pushes away any bacteria that the alcohol failed to kill).
Then inject the needle at about a 45-degree angle – no need to pinch the skin. Fully depress the plunger, pull out the needle, and you're good to go.