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.
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 you 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.
When selecting a coach, consider what is more important to you - promoting and getting more revenue from gate receipts, or establishing a strong training program. Select a coach accordingly... the best two options are probably Inadequate trainer with Passable Leadership (for the ambitious, win-seeking) and the Passable trainer with poor leadership (for those who don't mind sacrificing a few losing seasons in the beginning to experience long-term success). You'll probably want to upgrade your coach in a season or two anyway, so keep this in mind.
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. You may even like to start a relationship with those you trust and learn from their experience.
Manage your expenses. When you start out, less backroom staff is better than more. 1 Goalkeeper Coach, 4 Assistant Coaches, 5 Spokespersons and 1 Physiotherapist is a good start if you're training outfielders. For goalkeeper training, the number of Goalkeeper and Assistant Coaches could be reversed, but you'll probably want at least two assistant coaches to maintain your outfielders' form.
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! Stay on top of your economy, you won't believe how easy it is to bottom out if you go a few hundred thousand in the negative.
Midfield is the most prominent sector on the field. The distribution of regular chances during a match is based on ball possession, which in turn is based on midfield strength of both teams. This doesn't mean dominating midfield is the key to success. You will need a well-balanced team capable of both creating and scoring chances, as well as defending against your opponents attacks. However, it is especially important to establish a strong midfield and a few good forwards in the bottom divisions.
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 MOTS (Match of the Season) everything else). You may read about a "clown" or "anti-clown" ("Nasse-setä") affecting TS as well; their effects are disputed — see the clown and anti-clown pages for more details.
Simply put, if you want to have prolonged success, you'll need to manage TS. To get the maximum benefit, you need a coach with high leadership so you can better maintain your TS from week to week. TS naturally gravitates toward composed, and is reset during the off season. Learning how to manage it can make the difference between promoting and finishing in the middle of the pack.
Understand that Hattrick is a manager game and it has very little to do with actual balls or grass. Being a RL football trainer won't help you at all in your Hattrick career except very slightly with choosing tactics. Like Clinton said; It's the economy, stupid!
Play 'Normal' (Ie, no tactics like AIM, etc...). Only start using tactics if you know how they work and if they are necessary. Until you have an understanding of formations and a basis for tactics, you probably won't want to do anything but normal for a while.
Scout your opponents. Learn where they are weak and strong, and know your team.
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, and consider using a Manager Program (see Helper Applications or Category:Helper Applications) to choose your lineup.
Understand that there is a random factor at play. Expect unexpected results... they are part of HT-life.