Encourage good costume.
Reward players/crew who are in costume. Mordavia* based 1/3 of the character XP on costume. The absolutely most simple piece of costume anybody can make is a T tunic. Someone who knows how to sew can make one in 10 minutes. Here is a tutorial on making a T tunic. T tunics are about fifty times better than a tabbard because you can throw a belt around them with a pair of black jeans and about the only thing that looks out of place is your footwear.
The rubber monster masks can work really well with a helmet or a cap, but with the back of the head uncovered they look like a mask.
Start the game when people get there.
As a GM, you may need to brief every player and monster. When you’ve finished briefing each one, send them in. Ideally, you’ll be sending in some monsters first and they can have a bit of a poke around and an explore to become familiar with their “home”. If half the players are late, the early ones can scout around a bit and take more direct action when they’re reinforced later.
At Mordavia* the crew turned up early and set up and the players turned up whenever they wanted to. They got changed in the carpark and walked the gauntlet up to the tavern where they met up with the other characters. The monsters didn’t get dressed in front of the players.
Encourage teamwork - run workshops - build shit
Costumes take time and money to make. It’s a cruel law of economics that people with time have no money and people with money have no time. However, once you get a group of people you’ll find that between them you have time and money. Get a team together and build shit.
At the real “quick and dirty” end of the costuming is the XIIIth Roman Legion. Steph and I built a Roman Legion for about $100-$140 - the workmanship on all my stuff is horrible (Steph’s sewing was better). We may have spent 20-30 hours building enough costumes for a legion of eight people. I’m not proud of the quality of the costumes. But in terms of the time and money invested in them, they were a success. They also killed a huge number of players, but not on my watch 
The Guard were another success in terms of a team effort building a unit. Matching shields, heroic armour and cloaks and a combined effort added a huge amount to the game. The sense of camaraderie you get when a group of three or more people start a project is fantastic. The pride when you take the field as a matching unit is worth the effort. (“Leave none behind”)
Reward Excellence
When people contribute above and beyond their peers, reward them. If someone has run a whole bunch of games and spent hundreds of hours building props, weapons and costumes - reward them. Do something cool in game like knighting them or opening a new college of magic to them. Other players see the benefits of contributing and they contribute more as well.
I’m not suggesting you get as mercenary as some MMORPGs and start selling wands of fireballs on eBay, but don’t just let peoples contributions go unnoticed.
- I had very little to do with Mordavia. I arrived when the game was mostly finished. I crewed in 3-4 games and played in one. It worked.