There is also a trick that someone posted on here in using the syringe. Once you have the right spot and have pushed the needle in, before you push the plunger, back the syringe out slightly. Maybe a 16th of an inch, I started doing this about a month ago and have never "missed" since then.
I would also suggest that you go through all the old postings about Injections. Every guy is kind of different and so many have discovered that they need to do variations/combinations of ED meds and Trimix. Trimix works extremely well for me so I haven't tried the half of a Viagra/Trimix combo approach.
When I tried Caverject and Edex, I would get the 4 hour erections. Never had to go to the ER, but the alprostadil was really painful.