Rosewood simply is a bit too soft to endure most bass strings, no matter how gentle you play. But different types of string cause very different amounts of abrasion:
- Standard roundwound strings are worst, they'll eventually dig even into an ebony fingerboard. Use them only if you want the brightest sound possible, and are willing to take sacrifices.
- Completely smooth strings like those normally used on bowed string instruments are great, they don't cause any notable wear at all1. You find them as tape-wound. Many people consider their sound not particularly exciting on fretless electric bass, but it's a matter of taste.
- Half-wound strings OTOH, or sometimes even strings sold as "flatwound", even though they lack the thick bulges of roundwould strings, will often still have rather sharp "edges". They can thus leave marks on the fingerboard almost as quickly as roundwounds do, while sounding much duller. I therefore don't consider them very useful.
- My personal favourite: hard-plastic coated, roundwound strings. They're softer than all-metal strings, but don't feel soft, nor slippery (though somehow still particularly "smooth"). They sound a bit less bright than plain roundwounds, but not nearly as dull as flatwound types.
- There are a couple of exotic strings I haven't tried myself, though it might be worthwhile. For instance, nylon-flatwound, which I understand sound pretty great and cause no wear at all. Also there's weird stuff like massive-rubber, but that's only for piëzo pickups and won't sound like electric bass at all.
The difference between those string types is, I think, bigger than any difference your playing technique can make, so carefully choose your ideal strings!
The only other thing that can significantly protect your fingerboard is to give itself some hard coating. Good epoxy should help; I haven't tried it, but I can't imagine I would like the feel. Again there are more exotic options like stainless steel (mostly "popular" on fretless guitars), but I suppose that's hardly an option for you.
Regarding playing style – if you develop a "lighter touch" for the left hand then at any rate it can't hurt: it's always optimal to use only as much pressure as you really need, just a matter of efficiency.
1Violin through double-bass fingerboards still don't last forever, but the reasons are different – mostly, the wood is harmed by sweat from the fingers, starts "buckling" and thus needs to be re-straightened.
