I invite them to my home for Shabbos meals, to start.
When I was in College, I used to crochet Yarmulkes (so the frum kids would know that I was one of them), and I made over two dozen for guys who promised to wear them if I made them. Out of the dozens, I know for a fact at least 7 who not only wore them, but are frum today (I do not take credit for it at all – these guys started to come to Jewish-oriented events at school after they started wearing the yarmulkes, and someone good in kiruv chaaped them).
the best way to be mekareiv someone is by example. When someone OTD or never-frum, sees a frum person being a really good guy, being a wonderful neighbor, NOT being judgmental, it opens up some emotional floodgates in them that can lead to kiruv. When I had a neighbor who was sitting shiva, though not frum, I cooked a meal for her, and arranged for other neighbors do the same. Maybe she will never be frum, but she has a lot more respect for frumkeit now than she did before. Small steps can lead to traversing miles.