What are some intimidation techniques for the Pyro?

As a pyro, your job is to light people on fire. Sounds simple enough. Here's a few ways you can be even more effective in doing that.

Ambush

You seem to be under the impression that just because a class has more health than you that you can't spread fear into their hearts by lighting them on fire. This is incorrect - every class hates being lit, and will seek out health as quickly as possible ot put out your flames. And if they don't, they'll be soaking up a lot of damage due to your flames (more reason to always target the Medic first - flames mean nothing if he's there to lend quick heals to everyone).

Damage Boost

Flames are good, but they'll hardly eat through a Heavy's tremendous tank of health. Which is where your guaranteed-crit weapons can come in handy. The Backburner, the Flare Gun, and the Axtingiusher all offer a damage boost of some kind - but you'll need good maneuvering to get the most benefit out of it. The Backburner is good if you can already launch good ambushes and get behind enemy lines, the flare gun is good for picking off retreating foes and doing additional damage to tougher classes, and the axtinguisher is good if you're very good at ambushes and need to take down a very tough target on your own.

For the Love of God Airblast

Pyro is one of the best zoning classes in the game. Their airblast ability can force enemies back or into corners where they're easy to tear apart. They can push high-health classes right into hazards or off cliffs where they'll no longer be a problem. They can reflect or push back nearly every projectile in the game. The airblast is an indispensible tool that you should never forget to use. And lest we forget, it can completely negate the effectiveness of an ubercharge when applied correctly.

You won't need any fancy tricks if you can master these things - a powerful Pyro is intimidation enough for a team. Make them respect your ability to appear out of nowhere, deal damage, and completely destroy their team cohesion, and you've done your job.