Stay In Season
One way to stay within your budget for flowers is to choose ones that are in season for the month of your event or wedding. David Tutera put together a great resource on Bridal Guide for flowers in season for each month. If you don't like the flowers in season for that month then talk with your florist and explain your budget. In most cases, they will work with you to take items out or revise to stay within budget (as long as it's reasonable).
Fake It When You Can
I am not a big fan of fake flowers, but in some situations they can look very real and save you a lot of money! For instance, you can make a fake orchid look very real by submerging it in water in a simple cylinder vase and adding a floating candle.
Another tip for making fake flowers look real is to cover up the stems. The green plastic stems are a dead giveaway, so covering them up with ribbon or in a vase that is not clear will make them look much more real.
Use Your Grocery Store or Local Produce Store
If you've already hit your floral budget, but want to add some flowers for the cocktail hour or other areas, consider having your planner or family friend pick some up at the grocery store or local produce store to fill small vases. One bride that I worked with had bud vases to put a single flower in for the cocktail hour, so I gladly offered to stop at the grocery store on the way to pick up some. The flowers were only $10.00 and a quick trip on my way to the wedding. Plus they looked beautiful! You can sometimes find bud vases at the dollar store or consignment/vintage store for very cheap.
If flowers are way over budget, then consider candles for decor. Floating candles are beautiful for evening receptions as are pillar candles and can create a romantic feel to your event. Sometimes it can be just as expensive as flowers to rent these items though, so you work on purchasing throughout your wedding planning process at your local craft store with coupons or online (watch for shipping costs!). You can incorporate a combination of both to fit your decor budget and create the event atmosphere you envisioned.