Instructions
Familiarize yourself with all the Tetris pieces so that you know where to use which ones. The long "I" shaped block is the most coveted one, as it is the only block that can clear four lines (the maximum) at once, but every piece has its place.
Get into the habit of making your blocks drop faster (the "hard drop") instead of waiting for them to drop on their own. Not only is this worth a lot more points depending on the version of the game you are playing, but it will also enable you to cope with higher levels during which blocks drop quickly on their own.
Line your blocks up in such a way that there are no gaps in between them. This is vital to getting more points. Lines that have gaps between them cannot be cleared away easily using the "I" shaped block, so you will lose points clearing them away line for line.
Stack your pieces in such away that you have channels on the far left or right of the board where the "I" shaped block will easily fit into. This will give you the most points and more space to work with while you wait for the "I" blocks to appear.
Pay attention to the Preview pane if the version you are playing supports it. This pane will show you what piece is coming up next and allow you to plan your strategy accordingly. If you know what to expect, you can set up your pieces in such a way that you get more points when clearing them.
Play on the highest level that you feel comfortable with. You get more points for playing on higher levels, so if your version of the game supports it pick the highest number you can handle. Doing so won̵7;t get you to the next level quicker, but allows you to play on the selected level longer. The level will still only increase once you have the required amount of lines cleared, but you'll get many more points along the way.
Perform "combos" by clearing at least one line for every block that you place. The ideal combos are through the use of the "I" shaped blocks, but if your stack is high enough you can still earn points by clearing away lines without interruption using the other blocks. If you place a piece without clearing a line, the combo will end.