  Using a dynamic functional renormalization group treatment of driven elastic
interfaces in a disordered medium, we investigate several aspects of the
creep-type motion induced by external forces below the depinning threshold
$f_c$: i) We show that in the experimentally important regime of forces
slightly below $f_c$ the velocity obeys an Arrhenius-type law
$v\sim\exp[-U(f)/T]$ with an effective energy barrier $U(f)\propto (f_{c}-f)$
vanishing linearly when f approaches the threshold $f_c$. ii) Thermal
fluctuations soften the pinning landscape at high temperatures. Determining the
corresponding velocity-force characteristics at low driving forces for internal
dimensions d=1,2 (strings and interfaces) we find a particular non-Arrhenius
type creep $v\sim \exp[-(f_c(T)/f)^{\mu}]$ involving the reduced threshold
force $f_c(T)$ alone. For d=3 we obtain a similar v-f characteristic which is,
however, non-universal and depends explicitly on the microscopic cutoff.
