A slightly different technique, if there might be a small enemy army near, is split your army and keep splitting it down as far at is safe.
This would be pressing the 's' key, rather than pressing the 'd' key twice (assuming hotkeys are the same in most languages).
Beginning with one 32 regiment army,
It can split into 16 regiments (besieging 2 provinces at once)
They can split into 8 regiment armies (besieging 4 provinces at once)
They can split into 4 regiment (besieging 8 provinces at once)
....and keep going as long as you think it is safe...
Advantages:
- You can increase the amount of besieging armies very quickly.
- Attrition is unlikely to bring your armies under 1000 soldiers (so long as you're not down to one regiment armies).
- If a small stack of enemy troops appears, your occupying forces are more resilient.
- Merging two occupying armies is more likely to result in a reasonable battle-force.
- Smaller likelihood of running out of infantry in the main battle-force (and getting insufficient support)
Disadvantages:
- Lots of micromanagement.
- Attrition for occupying forces can be higher.
- Doesn't work as neatly if you don't scale your armies by a factor of two: 16, 32, 64 (though 12, 24, and 48 regiments also can work).
- Doesn't leave you with a single battle-force to combat very large armies.
- The smallest and more divided forces (2, and 4 regiments) may have poor compositions for battles.