Like everyone is saying, friends help. But, if for some reason they aren't available, they kinda suck and aren't a challenge whatsoever, or there's another problem, use CPUs. While it takes a while, a good way to familiarize yourself with your main/whoever you're currently using and the rest of the characters as foes is to brawl every character seperately, gradually increasing the levels until you've beaten them at Level 9. And yes, that means one level at a time, not just going to Lv.9 and beating them, as it is true that lower levels have more human reflexes that compare more to actually playing somebody. However, 9's are also good practice since to beat them you need to think faster than usual, come up with better strategies, and excerscise a bit more skill. If you want to practice specific techniques, use Training Mode to learn how to actually do it, then start brawling CPUs at low levels to try it out.
But, between your training sessions, it would be a good idea to be looking for a way to get WiFi. Seriously, CPUs are nothing like real people, and you're more likely to find serious Smashers on WiFi than with friends.