FEHB plans come in two basic flavors: those available nationwide and those specific to your region. Then there is the fee-for-service -vs- HMO/PPO choice within each of those groups (though nationwide plans tend to be fee for service. If you already have doctors you want to keep using, the first thing to do is find out which plans they do or do not accept.
When you've got a list of plans you want to compare to each other, comparing costs is pretty straight-foward but a bit tedious: what you need to do is add up the total cost of your share of the premiums + what you expect to pay for copays. Just make yourself a list of the drugs you use and doctor visits and look up your copay under the competing plans. Take note of plans where prescriptions via mail order let you get 90 days worth for just 2 monthly copays - it can be a deal-maker. The copayments associated with hospital admissions are often the main difference between plans: plans that make you pay a percentage of the "approved bill" will almost always cost you more than those with a fixed fee per day of hospital time. The other thing to look at is the clause that limits what you pay out of pocket in a given year - all plans have it and all of them that I've read make it tough to hit the limit by excluding the prescription copays from what counts toward the limit...but it is a factor that can be relevant.
When you've got all these costs added up you can see which plan will cost you less money given the assumptions you used and can also see which kinds of costs can make or break the deal.
The non-money part of deciding is whether the plan gets good reviews from the doctors/nurses for approving things you might need - just ask your Dr. staff their experience. The last thing I can think of is whether a plan requires referrals or not - most HMO/PPOs require referrals and it can be a pain and an extra expense to get referrals from your "primary" all the time.
Don't forget to sign up for the flexible spending account at the same time - they take money out of your pocket up front but it is untaxed and you get it back in your pocket when you spend it on copays, etc.