Position: If you have the right cards (let's say AA for example, or T2 if you are Doyle Brunson) but you don't have the right position, you are in trouble or at least in a predicament. It´s not just saying that having the button or being close to it is good, and the rest is bad, but it's always key to consider the position you have in the hand before jumping in. Never underestimate your position, it could cost you lots of money.
Hand selection: When you start playing poker for fun, you start liking some hands combinations more than others (for example, I like TJs), but that all ends when you take poker more seriously, and after that, some players decorate their offices with big tables with initial hand rankings. Hand selections consist in considering the position and the initial hand you have and choose whether you want to call, raise, or fold.
And many other concepts (new ones emerging every day), but we are not machines, and hundreds of mathematical calculations in several tables at the time are just not possible even for the best players. So, then the specialized poker software joins the party. Unless you can handle hundreds of simultaneous calculations or you are the "21" movie main actor, you will need a poker software to improve your results.
* Note: of course, if you are playing in private reservations where support software is forbidden and the field is just so insanely weak, then the strategy is different and usually playing at 32 tables at the same time is not possible.
There are two main poker software categories:
Online poker software or trackers: This type of software allows the poker players to collect information from all the tables they play in, and shows in real-time statistics of the opponents in the form of a HUD (Heads-Up Display). Hundreds of statistics are available so one of the most important tasks of the user is to customize the HUD according to his needs, showing only the relevant information to make the right calls. Hold'em Manager and PokerTracker are the most popular tools but not the only ones, as new and advanced tools have emerged like DriveHUD and Hand2Note.
Offline poker software: software for those who deeply study the game. With offline software like SimplePoker or CombCalc6Plus (for 6+ Hold'em players), you can simulate every possible situation at the poker table to come to the right decisions, which in the long-term will result in a positive EV to you.
There is other software on the market, of course, hand converters which enable HUD where is not fully supported by the room, table selectors to look for fish at the tables before sitting down and many other tools.
Besides saying the usual "it's important to have a strategy and use a poker software if possible", we'd rather say "enjoy the game". It's like the popular saying "choose a job you love, and you will never have to work a day in your life". Understanding the poker mathematics, it's not easy and of course, if you don't enjoy the process, it doesn't matter if you have the best HUD of the market or the best poker coach, poker is just not your thing. Learning, perseverance, commitment, and love for the game are the poker key success criteria.