Keep your old kitchen cabinets, but give them a new look. Repaint or refinish them instead of replacing them. Sand all surfaces well before painting or refinishing. Add molding or trim to dress up the cabinets.
Update your color scheme. Paint your walls, and buy coordinating artwork, accessories and throw pillows for the sofa. Keep expensive large pieces, such as a sofa and custom draperies, in neutral tones, making it easy and inexpensive to change the look of a room by adding pops of a new color.
Buy new lighting fixtures. Add task lighting to a kitchen, a new pendant fixture over the island or new exterior fixtures to the front porch or over the garage. Updating the lighting makes a big impact at relatively little cost.
Use laminate flooring instead of hardwood flooring. Laminate flooring gives much the same look at a fraction of the cost.
Get estimates from multiple contractors if you plan to splurge on a major professional project. It pays to shop around.