As most of you know I fly over 150,000 miles a year (did over 200,000 last year and 186,000 on Delta alone) and have flown over 1 Million Miles on Delta alone. I have been in the back probably 50,000 of those 1 Million Miles.
The secret is keep your flying on 1 airline (and 1 alliance) alone and build status with that airline. Which airline you choose has many factors to look at and are based in large part to where you're located.
Here's the logic I used based out of Buffalo.
First off, I had to pick an airline to fly. I chose Delta, because looking at the competition, here's what I saw.
American - had to connect ALL flights through ORD only. They only offer regional jet service. This can really screw things up if ORD is down as there is nowhere else to go. The regional jets that AA fly's don't have FC (at leas the crj's they fly into buf), so AA was a no go. Frequent flyer program 25,000 miles is low level, 50,000 miles is mid level and 100,000 miles is top level. For low and mid level you're given "stickers" which you can use to upgrade flights. You earn a certain amount of stickers based on how often you fly and you can also purchase extra stickers. At 100k miles you get unlimited upgrades.
United - Offer flights again to ORD, but also offers flights to Washington DC. The flights to ORD can be in full size jets, but the majority of the flights are Regional. All flights to washington are regional. Full size jets offer Economy + seating which give you a lot more room if you're in the back of the bus. Better than AA but not the best choice. Going to be merging with CO soon. Frequest flyer program has 25,000 , 50,000 and 100,000 mile levels. They also offer a "secret" level (global services) that is earned based on SPEND, not on miles flown. Global Service peeps don't sit in coach. Their top level program is good as well (1k) but they can spend time in the back. Having global service really is a better way for most people, not me, as I'm always looking to save a buck on tickets so this isn't the best program for me.
Continental. Offers flights to Cleveland, Albany, Hartford and Newark. Again, most of the flights are regional (and Albany and Hartford are prop jobs), but some are full service. Merging with UA shortly. Frequent flyer program 25,000, 50,000 and 75,000 level. Problem is by having 75,000 as your top level there are WAY too many people on the top level which makes getting upgrades more difficult. Considered to have the best hard product in the industry, however many reports of Platinums (their highest tier) sitting in the back too often. They also have FCM. FCM is FIRST CLASS MONITEZATION. I'll go over this more later on, but they offer people a chance to upgrade for cash and those upgrades can compete with platinums.
USAir ... well, it's more like US Scare. Terrible company and their Phily HUB has the rudest people in the world. Offers flights from BUF to PHL, BWI, PIT, LGA and maybe a few more. If you're a HUB captive then I guess its ok if you can get to Chairman Preferred (100k miles a year), but if you're not chairman you'll sit in the back often.
Southwest - Southworst. The airline of choice for Ma and Pa kettle. They don't even have FC - out of the decision.
Jet Blue - good product but again, no FC, leaves them out.
Airtran - being taken over by Southworst and when they do they are getting rid of FC. Rules them out.
Delta. When I started they offered the most flights out of BUF on full service jets and to dat ethey continue to do so. Decent amount of choice to connect through so if there is weather problems in one area then I can change my flights and go through a different hub. Very, very few Diamond medallions (word has it there are less than 20,000 diamonds in their whole system), so I get upgraded just about everywhere I go. And oh, I am the #1 flyer out of BUF on Delta. Yup, last year I flew more on delta out of buffalo than anybody else. I get taken care of big time by them. Love em and the right choice for me.
Now, I am not in a HUB, so other than going to a few cities I'm going to have to connect on just about any trip. This can be a pain, but it also BUILDS miles. Every time I go anywhere I connect - either in JFK, Detroit, Minny or Atlanta. If I go BUF-DTW, although I'm only flying 241 miles I get credit for 500 miles as Delta uses a minimum of 500 miles for every flight you take. If I go BUF-JFK then I again get 500 miles but fly 301. If I go BUF-ATL I get 712 miles. Then the connecting flight gets me to my final destination. If it looks like I'm going to have poor flight year (not many flights), in order to keep my status (Diamond, the highest you can get on DL), say I'm going to LAX, I'll fly BUF-JFK-LAX, which actually takes me out of the way to get the extra miles (it also helps that Delta offers Business Elite on the flights from JFK-LAX, JFK-SFO, which is more like international first class than domestic first class).
So, after you choose your airline keep flying that one airline. People out there even take what's called "mileage runs" - flights to somewhere and back JUST TO EARN miles (or keep status).