Original Ten Principle Approach
Read and understand the rules!!!! You cannot expect to be good at this game without a basic understanding of how it works. The rules are VERY GOOD and should be read more than once.
Put in place a strong training system.(Ie, train one primary skill for at least 3-4 seasons, and train 17-19 yo players) (trainees should be sold when they get older for profit, and replaced with new youngsters, as older players take longer to train) This is how you will improve your team and make money. (NB. Always play a formation which maximises the number of players who receive training. (Ie, play 3 forwards when training scoring)) Play different players in mid-week games than in your league games, and always play a mid-week game. This doubles your number of players being trained. Train at 100% intensity.
Make a plan and stick to it. HT is a marathon, not a sprint. (Always, however, learn from the forums and tweak your plan where it needs it. Learn who is trustworthy for advice on the forums and who isn't. Microscope is.)
Manage your expenses. When you start out, less backroom staff is better than more. (1 goalie coach, 5 assistant coaches, 3 spokespersons & 1 Physio(maybe) is good to start(if you're training outfielders))
Only spend money on anything if (a) you can afford it, (b) if you really understand why it is necessary to do so and (c) if you can spare the cash. You DO get charged interest if you're in debt, 5% every week. Bankruptcy isn't forever, but it's annoying waiting for a new team for a few weeks!
When upgrading a position (usually after the sale of a trainee and after the trainee replacement has been purchased), replace a player with a player that is 2 skill levels higher.(No more, or less. Teams with a single 'star' player can be predictable, or the 'star' can have a bad run of form, and thus they're easier to beat)
Win midfield. Although balance is desirable, midfield is still King.
Manage your TS (Team Spirit). Why? because it affects your midfield lots. (In it's simplest form, this means PIC (Play it Cool) games you can't win OR can't lose (NB. Principle 10), and Normal or MOS everything else) NB owning a "clown" does not affect TS significantly. The leadership of your trainer is far more important.
Scout your opponents. Learn where they are weak and strong, and know your team. ("Know yourself and know your enemy, and you will never be beaten in a thousand battles." Sun Tzu)
Understand that there is some random factor at play. Expect unexpected results...they are part of HT-life.