If you are away for less than three weeks, you can vote without any problems. You can vote by post either before you leave or when you get back, if that is still before polling day.
Of course if you're back home on the weekend of the vote, you can go to your local polling station and cast your vote in person.
Within Switzerland
You can ask to receive the explanatory voting brochure a little earlier. Please contact your commune for details.
It's not possible to receive the ballot in advance.
If you're away for a longer period, you can have your mail forwarded. The Swiss Post offers this service for a fee.
Abroad
You can ask to receive the explanatory voting brochure as soon as it's available.
The authorities will send you your ballot up to five weeks before the vote. However, if you go abroad earlier than that and won't return before polling day, you can neither receive the voting papers nor vote.