If a default shipping mode other than default is desired, enter it into the DefaultShipping directive:
DefaultShipping upsg
This will make the entry on the order form checked by default when the user starts the order process, if it is put in the form:
<INPUT TYPE=RADIO NAME=mv_shipmode VALUE=upsg [checked mv_shipmode upsg]>
To force a choice by the user, you can make mv_shipmode a required form variable (with RequiredFields or in an order profile) and set DefaultShipping to zero.