You block a garment every time you wash it, but only the first time is the full job with pins and a tape measure to match the schematic. Once the shape has been set, a wash and a flat dry are enough for the stitches to settle again.
So after the first time: hand wash it, as the fiber allows, press the water out with a towel and lay it out to dry. A rinse free wool wash is useful here, since you cannot agitate the garment to clean it.
Lace is the exception. A lace shawl wants the wires each time, so the lace opens up again and sets.
1 Block again every time you wash
How often will you block a garment? Probably every time you wash it. The stitches need to set again after each wash.
2 Skip the pins after the first time
The full process, with the pins and measuring to the schematic, is for the first time only. After that, hand wash it as the fiber allows, press the water out in a towel and lay it flat to dry. The shape was set the first time, so there is no need to stretch and measure again.
3 Use a rinse free wool wash
A rinse free wool wash cleans the garment without agitating it, conditions the fibers and makes it smell nice. For a lace shawl, you will want the wires again so the lace opens up and sets properly.