Two additional tips:

1) Don't focus exclusively on accumulating airline miles. It can sometimes be difficult to get award seats (due to capacity restrictions) and some airlines (generally not the US ones, though) charge significant fees on top of miles for award seats. It's worthwhile, therefore to focus some of your efforts on accumulating points with a hotel's frequent guest program, since several of the major hotel chains have recently eliminated capacity restrictions on award bookings and generlly do not charge any fees on top of the points for free nights. (They have devalued their points, but that's another story.) Accumulating a decent stash of hotel points means that you'll have a couple of options when you want to plan a trip using miles and/or points. You can try to find reward seats, but if that's not possible or isn't a good value, you can look at redeeming some hotel points for one or more nights in your destination of choice. This can be a good strategy particularly for popular destinations where: a) reward seats are scarce; and b) good hotels are expensive.

When choosing a hotel program to focus on, think about which chain a) has enough coverage in the places you're likely to visit; and b) has a range of hotels in a range that extends down to prices you (or your employer) would be willing to pay $ for AND up to the kinds of hotels you'd look forward to staying in on vacation. For example, personally, I wouldn't focus on Hyatt because i) it's generally out of my price range for vacation stays (so I'm less likely to accumulate points), even though ii) it is a hotel chain I'd like to stay in on vacation. Similarly, I wouldn't focus on Choice or Best Western, because even though there are many hotels in these chains I can afford, they aren't generally the hotels I want to stay in on vacation. (Although there are some great BWs in Europe.) Instead, I focus on collecting Marriott and Hilton points because they have a wide range of hotels worldwide, have hotels in the chain I can afford to stay in when using my own $ and also have high-end hotels I can splurge on (using points).

Because I have (or at least until recently) had a large stash of miles (with BA, Flying Blue and Aeroplan), I've focused most of my credit card purchases in the last year on Marriott and Hilton-affiliated cards. A number of FT'ers are also big fans of the Starwood Amex, because of its flexibility: you can convert SPG points into miles with a number of airlines.

2) Investigate "earnings malls" to boost your point totals. I'm not sure if airlines have gone into this in a big way yet (except BA and Aeroplan), but Marriott and Hilton have virtual malls, which are essentially internet portals that you click through to get to a regular retailer's internet website (e.g. click through the Marriott earning mall portal to get to gap.com). When you do this, the $ spent on the retailer's website generate points in addition to the points you get using your credit card. For example, if I click through the Marriott earnings mall portal to gap.com and use my Marriott Visa, I get 1 Marriott point per $ for using the credit card plus 3 points per $ for making the purchase through Marriott's portal. It's a good way to boost your point totals without significant additional expenditure (provided that shipping fees are lower than transportation costs for going to the store).