Here are some things that work for us, when Fidelio and I have to be apart:
We leave little love notes where they will be found. (On the mirror, inside drawers, on the coffeemaker, etc.; OR inside the suitcase/briefcase, folded into pajamas, tucked into the laptop, etc.). Some of them turn up right away, but some of them take days or weeks to be discovered, and it's a warm snuggly feeling every single time.
We talk on the phone several times a day, and always when we wake up and go to sleep. Maybe very short conversations ("Hi, sweetie. I'm taking a little break and missing you. How's it going? Ok. Gotta run. Love you!") but lots of them.
Sometimes we send gifts. It doesn't have to be long-stemmed roses, either. Ebay daily deals always offer free shipping and are usually cheap.
A couple of times when we were jonesing bad for each other, we've ordered the same dinner delivered and ate together on the phone. Domino's pizza is more or less the same wherever it's sold.
Just some ideas to get you started. I'm sure you'll come up with plenty of your own.