Are there any tactics for Bejeweled Blitz?

The primary answer here is speed, but there's more to it than that.

• Keep your speed up, making matches, so you'll continue to receive an ever increasing speed bonus. Your goal here is to get to the x1000 speed bonus where every match creates an explosion (and the gems flash) - but any speed is better than no speed at all.
• Make matches from the bottom up, there's more chance of a cascade
• Be aware of the multiplier gems - a match has to cause a cascade that removes 12 tiles (and more than 10s apart from the previous multiplier gem). So, make those cascades and grab the multiplier gems early.
• Don't plan out your next move too much. This is Blitz, remember? 60s is not enough time to ponder the inner-workings of cascades.
• Do look for more matches ASAP. That is, as soon as you locate a match and begin to drag it, start to look for the next.
• Remember that you have a Hint button - but that it takes a few seconds to locate, move to it, and click.
• Do purchase power-ups. With one of the latest updates, you can purchase powerups like multipliers in the field as you start, gem-swappers, and extra time. In my personal experience, the gem-swapping and extra-time are the most valuable to my score.
• If/when you do create a 5-in-a-row match and obtain a HyperCube, you should use it ASAP, on the color that you see the most of. This should create many cascades, up your speed bonus, and with fewer colors on the map, it should be easier to create your next match. (Remember that 5-in-a-row is the most valuable, so don't waste the opportunity on anything else).

Don't forget that if you really want to be a bejeweled-blitz-master, you'll need to do the following:

• Practice practice practice
• Watch YouTube Videos
• Read up other guides

I've played on facebook and my high score is ~1.2 million. Besides speed, the major factor contributing to super high score is the multiplier. A 8X multiplier at end game means you'll get 8X more score, which is more feasible physically than playing 8X as fast. Luck plays a big part too, I could play a perfect game with a couple blazing speeds runs but end up with low score because I didn't get many multipliers. However speed is the one thing you have almost complete control over. So go for very fast speed and make sure you clear those multiplier when you see them.

Speed it is mate. Try to go faster then the diamonds are destroyed and of course look out for 4 or 5 in a row which you give you bonuses where you can rest your eyes for a few seconds.