I see Ms767 has gotten you off to an excellent start. I would just add that you should familiarize yourself with your carrier's pass travel rules, and if you haven't already, read the "nonrev instructions, experience and miscellaneious" travel articles, here in the Forum. They'll give you a great overview.
Generally speaking, midweek, am (non peak) flights are your best bets. Now that doesn't mean you won't make the week-end flights, just that midweek flights offer the best odds; and when you factor in you're flying PHL-TPA-PHL (hub-sunny vacation spot-hub) and your low seniorty, you definitely need to maximize your odds. *you might want to invest in a good lucky charm*.

Back-up plan - you always want to travel with good back-ups, (alternate flights and alternate airports) because "it's better to be in the air than on the ground" and you never know when Mrs Newton's eighth grade will bump you to a later flight and you miss your connection. Trust me, knowing about that "red eye" that's goes into the regional airport located just outside of town, could mean the difference between calling in sick *cough-cough * and getting to work the next day.

Loads, ( as Mrs767 said) refers to flight loads, the percentage/number of seats occupied on a flight. Knowing there are 50 seats open on the 11am flight makes that ride to the airport with the kids, a lot less stressful *phwew!*.

Basic strategy
--Know your carrier's pass travel rules and non-rev boarding priority. I believe US Air's non-rev boarding is by pass classification and within that, seniority--meaning, you may need two lucky charms.
--Planning (flight loads, alternatives, weather)-- knowing what to do if that wide open flight changes to oversold, there's a snow storm in PHL or what you'll say if the gate agent asks "will you split your party".
--Be Flexible, sometimes you got to go back to go forward, so don't be afraid to take a roundabout route, remember --the goal is to get there.
--Don't check your bag--a checked bag means you can't make that last minute flight change.
--Smile, as Chris(nonrev1) would say, you get more flies with honey than vingear. Of course why you would want flies is beyond me, but it always pays to be nice .
--Don't leave the gate until wheels up, because you never know, that door could open up and on you go.
--On board don't relax until wheels up, because a late arriving pax could have you marching up the jetway in a New York minute.
One last thing, don't forget your airline ID, many hotels, car rental, attractions offer an airline discount.

Edited because of superfluous apostrophes.