Shallow-dwelling worms usually live in the top 12 inches of soil and randomly burrow throughout with no permanent home.

Tillage tends to reduce earthworm populations by speeding soil drying, disrupting their burrows, and burying the plant material they use as food.
The benefits of earthworms include:
improved water infiltration
soil aggregation
crop rooting

Depending on the location, crop fields can have from one to five shallow-dwellers (redworms, grayworms, fishworms, etc.) species of earthworms and perhaps one deep-burrower (nightcrawler).
The 3- to 5-inch long shallow-dwelling worms usually live in the top 12 inches of soil and randomly burrow throughout the topsoil with no permanent home. They eat residue and mineral soil as they burrow. The 4- to 8-inch long nightcrawler builds permanent burrows that can be several feet deep. They construct middens over the mouth of their burrows. The middens are a mixture of plant residues (their food) and castings (feces).

Tips
Here are some tips to maintain or increase earthworm activity:
Don't broadcast insecticides
toxic to earthworms in the
spring and/or fall when they
are active near the surface.

Rotate legumes with grass
crops. Earthworm
populations build up more
rapidly in a corn/soybean
rotation, compared to
corn/corn (or other
monoculture systems).
Plant a legume cover crop
such as hairy vetch in
continuous grass crop
systems.
Maintain soil pH between 6.0
and 7.0.
Avoid any tillage that will bury
crop residue and disrupt
burrows of surface-feeding
earthworms (nightcrawlers).
Add livestock manure to fields
to provide additional food and
stimulate earthworm activity.
Plant a cover crop in fields
where most of the crop residue
has been removed for silage
or bailing.

Here's an illustration of a nightcrawler burrow. Nightcrawlers build permanent burrows that can be several feet deep.

Limitations and suggestions
Management can increase earthworm populations on many soils. However, on some soils (for instance, very coarse sands and heavy clays with a high water table), no amount of management will help.
In fields where shallow-dwellers are present, changing management systems to something more suitable can increase populations in 1-2 years. In fields without nightcrawlers, there may be some benefit in trying to establish them if you're switching from conventional to no-till.
Here's a low-cost way to try (no guarantees):
Collect local nightcrawlers from
country roads or pastures on
rainy spring nights or mornings.
Protect the worms from the sun,
and place 4-5 together under
some mulch or residue in a spot
every 30-40 feet in the field.
Try to put the worms in the field
on a cloudy, wet, cool day or as
the sun sets.
Record their location in the field
and observe those spots for
evidence of midden activity over
the course of a year.