Coleus is an extremely easy plant to propagate and overwinter.

I have collected several cuttings from various coleus to overwinter at home.
Rooting in soil from cuttings is your best way of propagation.
When you choose your cuttings pick healthy stems, water your plant thoroughly a few hours before cutting, cut your stems at least 2" long using a sharp knife or other implement, plant your cutting in a good growing medium (vermiculite, perlite or sand), keep your growing medium moist, maintain humidity and do not fertilize. You should check after one week and transplanted when roots are two to three inches long.

Another easy,less time consuming, tried and true method is to take cuttings and root them in water. There is however a downside to this method. If rooting several stems roots can become entangled and be pulled apart. Try rooting each stem in a seperate container. When roots are two to three inches pot in soil.

Cuttings can be done at anytime. The best time however is early summer to have a good size going before fall. There will always be losses over winter so a good collection to start will get you through. Keep coleus in a protected sunny sight over the winter.

You can also bring your coleus in and treat them as houseplants until spring.

High Temperature:
Actual: 2°C Cloudy rain/wet snow 1.5mm Winds WNW27km/h P.O.P.40%
Average: 9°C
Record: 20°C in 1959

Low Temperature:
Actual: -5°C Mainly cloudy Winds NNW24km/h P.O.P.20%
Average: 1°C
Record: -9°C in 1951