Start by matching the type to your cleaning. Uprights, especially with a bag, do best overall on carpets. Canisters are easier to maneuver, particularly on stairs. Here's what else to consider before you buy:
Check the features. A brush on/off switch helps protect the finish of bare floors and avoid scattered debris. A motorized brush cleans carpets better than only suction. Also helpful: manual pile-height adjustment, which can be matched to carpets better than with automatic, and suction control for cleaning draperies and other delicate fabrics with tools.
Consider bagless carefully. Bagless vacuums save on the cost of bags but still require filters and need periodic cleaning and--for HEPA filter--replacing. And the dust and mess of emptying their bins is a concern if you have asthma or allergies, though even bags must be handled gently.
Try it out. Even if you'll order online, go to a store first. Push, pull, turn, and lift models you're considering. Check out the controls and features. If an online price is low, see if the store will match it.
Know about the noise. The noisiest vacuums we tested still don't produce 85 decibels, the level at which we recommend hearing protection, but any vacuum will seem louder in a room with a lot of echo, such as a tiled bathroom. Canister vacuums as a group tend to be quieter.
Watch for sales. Discounts of 20 to 30 percent off the price of even Dyson vacuums are increasingly available, especially during the holiday season. But for mass-market brands such as Bissell, Eureka, and Hoover, you don't have to wait till Black Friday. Watch store circulars for frequent sales of these and some other brands throughout the year. Better yet, sign up on manufacturer or retailer email lists for coupons and notification of upcoming sales and other promotions--at least till you've made your purchase.