Here are some general pointers on skills. We'll look at preferred skills for Strength and Agility teams as well as skills appropriate for both. Obviously if you manage to get a 'doubles' roll for the upgrade then you could look at the Agility skill suggestions for your Strength team and vice versa.
SKILLS FOR STRENGTH TEAMS
Stand Firm and Guard should be high on your list of priorities - and probably in that order. Mighty Blow does have a place in your team; that place being your punishing, unmoving Line of Scrimmage (LoS). Talking of the LoS, Break Tackle can throw your opponent if they're relying on some Tackle Zones (TZs) stopping your main Blockers from Blitzing their Catchers.
SKILLS FOR AGILITY TEAMS
Emphasising your strengths would mean choosing Dodge or Leap. Diving Tackle is a nasty surprise to play on anyone; as is Side Step. Although increasing your movement may seem unnecessary at first -- as you can run rings around many teams - skills like Sure Feet can gain players a fair burst of speed, whereas Jump Up can affect your players in a more subtle manner: it's great on those players who tend to start the turn on the ground. You really are spoiled for choice here as Agility skills are on the 'most wanted' list of every team.
SKILLS FOR ALL TEAMS
Block and Pro. That's all you need to know. All right then, so they're not the only good skills on the General Skills list: Tackle is a great indirect annoyance - as is Shadowing; Strip Ball has a rather specialist application, but forces skill choices upon your opponent. Finally you can teach your opponent to fear you by using Frenzy (remember to get Block!). If you play against Agility teams a lot then choose Pass Block for increased trouble!
For those that can choose Passing Skills Accurate and/or Strong Arm is a must to get an increased chance of placing the ball where you want. I would think hard before choosing between Safe Throw and Dump-Off and probably plump for the latter through personal taste - even though I have seen the most unlikely Interceptions happen!