For a gallery in which you will be hanging paintings or lightweight work regularly, consider a hanging system. Hanging systems stay mounted in the room. Hung near the ceiling, with hanging and movable wires, a hanging system can be set for any width or height. Heavy pieces, on the other hand, must be anchored in the wall. Anchors go into masonry or drywall and will hold a piece of artwork firmly. Anchors are especially recommended for wall sculptures or heavy glass pieces. Make sure the work to be hung has functioning hardware, such as wire or holes, so there is something with which to attach it to a hanging system or anchor.
An exhibition space packed with work can appear claustrophobic, while one that is too sparse can underwhelm. Work with your artist to find the right spacing for her work. Consider traffic flow, the size of the works and the architecture of the space. Big pieces need more room on either side. Floor sculptures or pedestals must have ample room about them -- you don't want one crashing down at the opening. For big group shows, packed floor-to-ceiling (or "salon style") hanging is acceptable to save space. Let your tastes and the specifics of the exhibition site guide you.
The lighting in an exhibition is critical, as it allows viewers to see each piece of work. Natural daylight, if available, is a wonderful source as long as it is not too strong; use diffuse daylight if you can. Skylights are ideal, and worth considering for a long-term space. In the event of strong and directional sunlight, use sheers on the windows. Use spotlights to highlight floor pieces and broader lights for overhead lighting. Aim to balance each source of lighting. It is fine, for example, to have a strong light shining on a painting but it can appear artificial and overly-dramatic if it is too strong.
The white box is the classic display space. Galleries typically use white to create a blank palette for the pieces on display. Museums, on the other hand, may not shy from strong colors like deep gray or burgundy for exhibitions. For some work, a little color on the walls can be fine and might even enhance the experience. Don't be afraid to go with a strong color, but remember to have enough light fixtures to counteract the increased light absorption of non-white walls.