Since the first Terminator failed, Skynet sent back increasingly sophisticated Terminators.
Another strategy would have been to send back multiple Terminators. Why was that not attempted? Was there a cost (e.g. power) to sending them back in time?
According to Kyle Reese, "Skynet's defense grid was smashed; we won." Their defeat was inevitable. Sending a Terminator back was a last ditch effort on Skynet's part to try and erase the leader of the human resistance from history. Skynet likely didn't have the time, resources, or enough troops in general, to send back more than one, especially considering the considerable power drain that it took to send back even one unit.
Assuming that Skynet was able to send back a whole army, it would likely run into significant resistance from present day human armies, which number in the millions and are backed by intact governments with fully functional industrial bases. Against a ragtag group of rebels, terminators were still being destroyed. Even without futuristic weapons, massive damage does affect them, especially when talking about airstrikes and other things such a massive force would be able to bring to bear against what is essentially an invading army.
This was essentially a stealth mission: the terminator is, as described by Reese, as an infiltration unit, designed to be mostly undetectable. Skynet obviously recognized this as a mission parameter & probably wanted the terminator to complete its mission as "quietly" as possible without anyone finding out about the technology used to create it, so sending one was much more advantageous.
The last point is the changing timeline. With each iteration, something changed. Just like the creation of Skynet was inevitable, so was the creation of John Connor. This didn't stop Skynet from trying, but it also knew that radical changes were much more likely to bring about the future that it was trying to prevent, rather than the future it was trying to bring about.
Alternative to wraith808's answer:
Each time a Terminator was sent back in the movies, the timeline shifted a little. Skynet got better technology, slightly earlier. As of the Sarah Connor Chronicles (the TV show), the timeline had shifted enough that the war was now in favor of Skynet winning.
It also showed that the kill-John-Connor-in-the-past tactic was simply one front on an infiltration war throughout the entire past century; from Skynet's point of view, he was no more important than anything else it was attempting to change in its favor.