The way I see it, when it comes to personnel there are generally three modes which can describe any NFL team. Usually, a team isn't precisely in one mode, but somewhere between 2 of them. Below I list the modes, followed by suggested approaches to the offseason for teams in that mode.
1. Rebuilding - This is the mode your team is in when the majority of your roster is filled with players who are ok at best. The few star players you have are mostly approaching retirement, and you lack depth pretty much everywhere. You're not likely to get a lot of wins and there won't be any talk of playoffs for you, (especially if Jim Mora is your coach!).
Offseason strategy - You need to get some star players, and a lot of just generally good starters. Most of your offseason moves should come from the draft. You're probably drafting high first rounders so make sure you get studs with those picks. The best player available approach makes sense for you and should probably be used all the way through the draft. When trading, trade to get more picks, not less. It's more important to get as many positions covered with NFL starting caliber players here than it is to try and get a superstar.
Ways to blow it - Sign aging vets to fill one or two holes while ignoring the rest. Try to get too many picks to rebuild with by giving away the store, (ie, trading all your good players and first round picks for a truckload of 5th-7th round selections).
2. Fine Tuning - You've got some talent on the field, but some holes as well. You lack depth at a number of positions and injuries can be devastating. You'll be able to generate wins with good coaching decisions, and may even make it to the playoffs. The luck of the game (injuries, bad/lucky calls, etc.) is going to impact your team's record most in this kind of personnel mode.
Offseason strategy - This is the hardest mode to strategically handle. You need to get your holes filled in order to move up to the next level so you generally can't just use a best player available approach since those players won't just happen to be at positions of need for you, (unless it's a couple of weeks ago and your name is Rick Speilman). Trading picks for veterans is often valid here, as is trading lower picks to move up in the draft since you don't need as many 'lucky crapshoot late round selections' as a rebuilding team would.
Ways to blow it - You need to have luck, patience, and a good sense of balance to keep from blowing an offseason in this mode. Going after too many aging vets or trading away a bunch of future early round picks are classic ways teams screw themselves up here. Everything that you need to do right in the other modes you need to do better here, (scouting, matching players to your scheme, etc.). The biggest key here is to avoid a "win now" mentality.
3. Dynasty - You did it! You've put together a team that has several star players, quality starters at every (or almost every) position, and good depth pretty much everywhere. Playoffs are all but guaranteed, provided your coaches aren't in the habit of making game-blowing decisions.
Offseason strategy - Keep your eye on the QB position. Always have a new QB in training behind your guy. Beyond that, you can almost do whatever you want and it'll work out okay for you. Of course, you'll want to fill the one or two holes that will pop up now and then, but beyond that you're free to do as you will. Best player available will usually work well here to get enough players to keep maintaining depth, and any players you bring in will have great veterans to learn from. At this point, since you've got all your bases covered, a win now mindset can be a good thing.
Ways to blow it - You'd almost have to blow it deliberately, or get really unlucky here. If you're going to screw up your dynasty, it will probably take more than some bad offseason moves to do it.
