I get on the Drop7 (web, iTunes) leaderboard pretty much every day. Today I was on it twice in the afternoon, although now, at the end of the evening, only one of my scores is still on the board.
Right side is daily, left side is all-time
You can of course get a massive, 70,000-point bonus by clearing the board.
After: 11,330 + 5 * 7 + 70,000 = 81,365
However, that's way too rare to form a strategy around (at least currently, for me). What I use to win is much easier: chaining. If you clear one number in Drop7, it's worth 7 points; if you chain that to two numbers, the first is worth 7 and the second's worth 39. The third will be worth 109, and it keeps going up from there. Here's a shot after a chain seven steps long; I got five numbers in the last step, each worth 907 points apiece.
In a sense, it's an investment game. You get random pieces, and the value of any piece can be anywhere from 7 points into the thousands or tens of thousands, depending on how wise and/or how lucky your use of the piece is. Your goal is to get the most value out of every piece.
I like to set up my numbers like one of these:
7 6 5 (anything)
7 5 (anything) 6
6 5 (anything) 7
You get the idea. Set the big numbers up at the edges, so that they can have horizontal lines which make them blow up. Then stack. Don't let the numbers blow up until you have a nice stack. In an ideal scenario, you would have six sevens, five sixes, four fives, and three fours stacked right next to each other, along with anything else filling up the remaining spaces. When you finally let the bottom row fill out, each seven would blow up, one by one, followed by each six, one by one, and so on down the line. You'd set up a chain this way and get huge points.
In practice, the randomness in Drop7 makes this ideal situation very unlikely, but the basic principle is easy to follow.
Here are some examples:
The last two got me 1701 points apiece
This example is NSFW, as it contains some golly darn foul language. This is actually an example of what not to do. I was pointing out that I had a five-high stack of sixes, accidentally touched the screen, which made it six-high, which made all the sixes blow up, which totally wrecked what I was trying to demonstrate. But take a look anyway, because it's still an example of how to stack; before I fucked it up, I had a great stack ready to go.
Update: I discovered a great Drop7 strategy guide.