You have to find your USP (Unique Selling Point) that differentiates you from the rest and gives you an edge beyond price such as (responsiveness, convenience, flexibility, additional services, etc).
Winning a bid is usually easy in comparison to sustaining the account long-term and making it profitable..
When you can, try to sell additional services such as the following:
- floor maintenance
- carpet care
- lighting (changing lighting fixtures)-
- special event/holiday/construction clean-up
- day porter services
- light handyman services (patching walls, painting)
- pressure washing
That way you will be in a much better position to counter other low-bidders and low profits margins since you'll have more work per account and be able to deliver more value to your customer.