Unless you buy higher fares at BA, don't even try to earn your BA miles by flying. 50% bonus for transfer from Diners Club typically at least once a year, 500 flat miles for even a one-day cheapo rental with Avis (but must use BA's AWD) when other airlines would only get you 50 miles, 500 flat miles for cheapo one-night Candlewood Suites stays (which with other airlines would earn you only 1 mile per $ spent), etc. Plus, of couse, all the usual partner activity (where in other cases you'll at least earn about the same BA as with other airlines).

And, depending on where in the US you are, it's a lot easier after you do one such upgrade. That's because to upgrade to C you have to buy a WT+ fare, and on the WT+ fare you earn well, and from the western US you earn about half as much back as you spent on the upgrade! Thus you don't draw down your net BA mileage nearly as fast as it would seem by using BA miles for such WT+ to Club upgrades.