Edge Garden Beds
If you don't have an edging material around the borders of your garden beds, use a flat spade to shave off clumps of sod to delineate the edges. You'll probably need to do this a couple of times, but if you don't you'll be fighting encroaching grass all summer.
Raise Containers Off the Ground
To allow good drainage in your container plantings, raise the pots off the ground or deck so water can seep out the drainage holes. This also will reduce the staining that can occur when pots sit directly on wooden steps or a deck. You can purchase pot feet or plant caddies from garden supply stores, or make your own pot feet using flat stones of similar size, rubber bumpers from the hardware store, or even old checkers from the game you never play anymore. Anything that will elevate the pot a bit should work.
Deadhead Early Bloomers
Once early summer perennials, such as peonies and foxgloves, have finished blooming, take the time to clip off the spent flowers to spare the plant the energy it would spend on forming seeds. That is, unless you plan to save the seed and do some propagating of your own. In that case, leave some seedheads until they turn dry and collect the seeds before the wind and the birds get to them.
Pruning Early-Blooming Clematis
If your clematis blooms only in spring, once it's finished blooming you can prune damaged and wayward stems, and cut back stems if you need to control the size of the vine. Leave the decorative seed heads.
Remove Moldy Strawberries
Harvest berries frequently and remove any that show signs of grey mold or rot diseases. Not only are these berries inedible, they quickly spread disease to other ripening fruits. Mulch under plants with straw to reduce contact with the ground where the disease spores reside.