Two things to keep in mind when you're running a campaign:
1. Missions should have objectives. When you're structuring your missions, always have a way for the players to win without having to kill everything on the map, and a way for them to lose despite killing everything on the map. Alternate these two fairly irregularly with the more normal, bog-standard "kill everything on the map" scenarios.
2. Keep in mind the Players' desires and work with or around them. Depending on the system you use, if you can get your players to give you 'hooks' in the form of back-story, do it. Tailor missions where they can shine. *(iow if you have a tanker or ASF player, it doesn't hurt to occasionally give them something only they can do properly, just don't overdo it.)
3. "Mook Forces"-have a few sheets pre-filled and pre-printed for the faceless mook types they'll run into, and a few 'key villains' on the side that you can advance as the players advance to give them 'boss fights' periodically.
4. be creative with your scenarios. Dig into the optional rules for things like environmental conditions, objectives to capture or destroy, and so on. The more actively your players have to think and engage with your campaign, the less likely they are to decide to sod off game day and go watch sports.