I have and still do play a lot of offensive teams, so I'll try to give some advice.
Tangerine hit on some good general rules, so I'll try to be more specific.
Your lead is very important. It will set the pace of the game. Things like Lum Berry Bronzong and Focus Sash taunt/SR/explosion Azelf are very good candidates, since they beat out almost every other lead (even if they have to explode to do it).
Where the rocks at? Set up Stealth Rocks as soon as you can. They will often ensure a 1-2HKO, and they stop focus sash, the bane of the offensive team.
Explode! Your team will likely have trouble with certain walls, and having a couple of explosion users really comes in handy. It's also a great way to turn an 11% HP poke into an extra KO.
Trick your opponent into sacrificing their important walls. If you are running DD Tyranitar, SD Lucario, and SD Garchomp, then it would be a useful idea to give TTar ice beam or Lucario HP Ice to get through Gliscor.
Resist Berries. The game will not last too long if all goes well, so in a lot of cases the resist berries are more useful than leftovers. Yache Garchomp is an obvious choice; Chople Berry TTar is a great Gengar killer; Wacan Gyarados will net you an extra DD (and a OHKO) against Starmie.
Stat ups. Dragon Dance TTar and Gyara will outspeed and 1-2HKO their counters after a single boost; Nasty Plot Infernape has so few counters it's almost a joke; SD Chomp is pretty much a staple.
Don't switch as much as normal. Offensive teams are ironically wrecked by opponents' stat-ups, so if you don't have a great counter to something it's often best to not let it set up.
Abuse abilities, resistances, and immunities. Tangerine touched on this, but it's so important that it must be stated again! Without proper walls you won't have a surefire counter to too many pokemon, so be sure you use pokes that have a lot of resists/immunities (e.g. Gengar, Lucario), damage reducing abilities (e.g. Intimidate, TTar's Sandstream, Sand Veil), and generally decent defenses (Garchomp, TTar, Metagross)
Sleep. It might as well be a KO. Though these moves are usually high risk, the reward is so great that it's tough to pass up if you can fit them in.
aaaaaaaaa Toxic Spikes. If you can fit a Toxic Spikes user onto your team, you will not be disappointed. Toxic Spikes help deal with opponents' offensive threats, in addition to providing predictable switches for you to capitalize on. As a sort-of bonus, offensive teams are generally very bad against toxic spikes, so having a grounded poison to clear them up is very helpful.
If I think of anything else I'll edit it in.