Try choosing your flight by checking cabin layout, like seatguru.com. It has a new flight-search function letting you filter your results by comfort as well as price, duration of flight, etc. Seatguru also tells you how much will it cost to trade up for a plane with more legroom or seat adjustment system.
If you want to buy a better seat, be sure it is worth it. Costs of trading up can range from $8 to $129 per flight, depending on the carrier and whenever you buy the upgrade. Sometimes price decreases as the flight date comes closer.
Spend your miles (if you have one) by upgrading but take note that there are some hassles sometimes. Like some cheaper fare economy tickets aren't eligible for upgrades, or require significantly more miles than full-fare tickets do. So often, you end up chipping in cash as well as miles.
Depending on the airlines, they charge differently based on fare class and length of flight. Also, always keep in mind your preferred carrier's alliance partners. You can often spend your miles for upgrade on their flights as well.