I like to use the tritone sub of the V7 chord, which is easier to find than the V7. In key Db, the modulation chord would be the tritone sub of Ab7. For me, it usually comes out to being a 13th chord. For the key of Db, it's:
D A C \ B C E Gb.....D13 chord
That may sound confusing, but once I got it down, it became automatic. You know how it goes...
In a progression, it might look like this:
C G \ B C E G .....Cmaj7
D A \ B C E Gb.....D13 (modulation chord)
Db Ab \ C Db F Ab.....Dbmaj7
This way, your fingers don't move around much! This modulation is a pleasing, jazzy sound!