This holds true whether you're pleasuring her clitorally or vaginally with your fingers or your mouth. It's a good idea to wait until she is very aroused before entering her vagina either with your fingers or your penis. Generally if she's not wet, she's not ready. It's as simple as that. If your lover doesn't have a lot of natural vaginal juices even when she is fully aroused, be sure to use a good silicone or water-based lubricant.
Nothing can be a quicker turn-off than rough, dry skin rubbing on soft vaginal tissues.
Water-based or silicone lubricant is better because oil can clog the sensitive vaginal tissue. The most sensitive part of a woman's vaginal canal is the first inch to two inches. It's here that most of the nerve endings are located, so when you first enter her concentrate most of your attention there. The elusive g-spot can usually be found in this general area, on the top of the vaginal wall, a couple of inches in. Imagine a glass lying on the floor.