Five things cost money when getting stuff printed:
   * Paper quality
   * Number of colours
   * Number of different things needing printing (posters, flyers etc.)
   * Amount of work the printers have to do
   * Urgency

It's generally worth using decent paper if you can - the posters and
flyers will look much more professional. If you're on a really tight
budget then black printed onto a cheap coloured paper is better than
nothing.

Each impression (colour) costs money. Two or more impressions costs a
lot more than one impression. Black tends to be cheaper than colour,
particularly for a single impression poster.

Posters printed black-on-white, then hand coloured with water-colours or
touches of highlighter pen can look very effective if they're planned
carefully.

If you need posters and flyers printed consider trying to put one poster
and two flyers on a single standard paper size, using the same colour
impressions. This means the printer can put a single run through the
press and cut it apart afterwards. This can give flyers almost for free.

The less work the printer has to do, the cheaper things will be, and the
more co-operative the printer will be the next time you use them.
Prepare everything yourself. Borrow a machine with a good desktop
publishing program and transfer your design onto it. Talk to your
printer and find out exactly what format they like - most want
camera-ready (full-size) copy, one original per impression. One thing to
check with them is how much to flare each impression (to avoid gaps
between colours).

If you're a perfectionist or you do much graphic design try and get hold
of a PanTone swatchbook, so you can define exactly the colour you want.

Urgency. Order stuff early. Even if it looks like the price will be the
same a week before the show as two months before the show. If you get
camera-ready copy to the printer early they'll be more likely to give
discount, either this time or next time. They'll also be much more
sympathetic in the future when you *really* need a poster in 24 hours.