Principle 1: 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.
Principle 2: Try to put in place a strong training system (e.g. train one of the Primary Skills for at least 3-4 seasons, and train 17-19yo players). Trainees could be sold for profit when they get older and replaced with new youngsters, as older players take longer to train. This is how can improve your team and make money. Always try to play a formation which maximises the number of players who receive training. (e.g. play 3 forwards when training Scoring). Play different players in mid-week matches than in your league games, and always try to arrange a mid-week match, as this doubles your number of players being trained. Train at 100% intensity.
A passable trainer with poor leadership is a good choice as a startup coach.
Principle 3: 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.
Principle 4: Manage your expenses. When you start out, less backroom staff is better than more. 1 Goalkeeper Coach, 5 Assistant Coaches, 3 Spokespersons and 1 Physio (maybe) is a good start if you're training outfielders. For goalkeeper training, the number of Goalkeeper and Assistant Coaches could be reversed.
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!
Principle 5: When upgrading a position (usually after the sale of a trainee and after the trainee replacement has been purchased), a good advice is to replace a player with another player that is 2 skill levels higher. 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.
Principle 7: Manage your TS (Team Spirit). Why? Because it affects your midfield a lot. In its simplest form, this means PIC (Play it Cool) games you can't win OR can't lose (nb. Principle 10), and Normal or PIC everything else). You might hear about a "clown" affecting Team Spirit in a positive way. Though a possibility, owning a clown has never been actually proven to be beneficial. The leadership of your coach is far more important.
Principle 9: 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) Stars are not the best indicator of where a team is weak or strong, as they can be deceiving. Look at match ratings for this.
Principle 10: Understand that there is a random factor at play. Expect unexpected results... they are part of HT-life. Try to avoid complaining in the conference with "Why did I lose?" threads.