Play against other players!~
I'm a firm believer in playing against other players to improve. A lot of players avoid multiplayer due to the high level of stress over stats. The simple reality is even the best of players sit around ~60% win/loss.
The very nature of the matchmaking is designed to pit you against players of equal skill and there is no shame in losing. While playing against computers may help you get your timings right you're missing out on a very integral part of the game itself: The Meta-game.
The Meta Game
The Meta-game can best be described as the current style of players and techniques in the game. When the beta started, 3 barracks Terran was the 'standard' opening. It wasn't until later that a solid 1:1:1 (barracks into factory into starport) was an accepted strategy.
This evolution of the game (fueled by the community) leads to dynamic strategies. Knowing and identifying these strategies is often the difference between a decent player and a great player.
Why its better to play against other players
By playing against a computer you lose the ability to identify and recognize an opponent's build when you scout him. These skills are essential in all levels of play! By recognizing the build a player can then 'expect' a certain army make-up or army size. Playing against a computer just won't teach you real player's timings, popular builds, and scouting.
Lets take an example: How does the community view TvZ on Lost Temple? Currently Terran can cause serious damage to most players by doing a 1:1:1 into thor and dropping it on the cliff overlooking Zerg's natural. Why is this successful? Currently Zerg has accepted that going for an early hatchery against Terran is relatively safe. This perspective (dictated by the Meta-Game) is already fluctuating! You just don't get this sort of thing against a computer.
An Insane Computer has built in advantages to negate your intelligence!
The enemy computer (on Insane) gathers at an increased rate over a regular player, making their strategies unachievable for regular players! Most RTS's do this! Because player's constantly evolve to beat the AI, the game developers program and advantage into the AI to help them be competitive. Enemy workers return 7 minerals a carry instead of 5, a 40% collection bonus. This 40% collection bonus leads to a larger army than ANY player can achieve at that time. This leads to bigger pushes with more units and loses the 'feeling' of an even game. In addition to that the Insane AI does not need to scout you. While the computer doesn't react to your build like another player, it also doesn't need to scout to find your 'hidden expansions'. If you only have one building left on the map the insane computer will walk to it directly (without scouting). This can be absolutely game changing! Against another player hiding a unit building structure (starport or dark-shrine comes to mind) can be the difference between a win and a loss if scouted!
These advantages lead to fighting a larger, and more educated (scouting) then possible army. This may help you improve your general mechanics: building placement, not getting supply blocked or early game timing. However, the actual game play is so dramatically different that you would be better off playing against any level of human opponent (even if they are no competition to you). Real players are much more aware of the evolution of the Meta-game (whether they know it or not) and practicing against REAL strategies is the only way to improve.
But Every Ladder Loss Feels Like Hades Ripping a Piece of Pride Directly Out of My Soul
There's a solution for this too! Find a practice partner to play custom games with. This will give you the ability to improve (and for them to improve) as both of you explore the game. But the real benefit to this is human vs human strategies without the stress of recorded ladder stats.
Hope this help!