I'm rather new to trading but have been a long time observer. It seems to me, solely through observation, that your chances go up significantly for a HTF pin if you have other HTF pins worth trading. Be mindful that trade values almost always fluctuate with time. I agree with Nathan, patience is the key in trading for a HTF pin. Make friends with fellow pin traders and frequent pin events and get active on Pin Pics. You never know when someone may cut you break or even have that pin but has it solely for trading purposes or obtained it from a previous trade but doesn't really care for it and you will be there to take advantage of it.
Also, it would help, if you can or are able to attend LE pin releases. For example, I have a friend who goes to the Disney Soda Fountain, buys pins (one for her and one to trade). Over time, depending on the pin, it's possible that the value will increase on that pin and it becomes more of a worthwhile trader that you may wind up being able to trade for your grail/htf pin. Recently, she was there for the surprise release of the Up BT pins. She got it for retail price (not sure what that was...I don't collect those ones) but turns out since it was a surprise and LE, it has sky rocketed in value and now she has a real bargaining chip she COULD use if she comes across her grail. Up and Tangled are currently the "hot high value" pins that usually trade well. Even if you don't collect them, it's worth attending a release event to get some because they are so sought after, they are usually highly trade-able.
As for approaching for a trade, just be brave and ask! Be very polite, friendly and understanding if they choose not to trade. I'd say 85% of pin traders are kind and will never snub you for asking/inquiring. As the saying goes, it never hurts to ask. I guess my tips are... patience, persistence and determination. Best of luck in your hunt!