With a new bed.
Remove sod, weeds, rocks, and roots. Till in 6 inches of compost or well-rotted mature for fertility and good soil structure. Get a soil test and amend the pH level (aiming for a neutral pH) if recommended.
Wait to work the soil until it is no longer soggy. Add amendments if recommended by a soil test. Use a digging fork or broadfork to gently aerate the top few inches of soil.
Throughout the growing season.
Whenever a bed becomes empty, spread a half inch or so of compost over the soil and rake it lightly into the top 2 inches. Then aerate with a garden fork.
Spread a 2-inch layer of compost or well-rotted manure on any beds being retired for the winter. No need to dig it in.
All the time.
Refrain from walking on the soil. Make beds narrow enough that you can reach into them from the paths.