I would not worry about any sort of "hit rate". Just concentrate on writing to people you like and admire, then the words will come easily. Write about what you know about. True knowledge stands out in a letter, and the letter writing stays fresh and fun over time.
Handwritten v typed is an old debate. Handwritten letters are more courteous, and show greater effort, but if you have crap handwriting like me, it can be hard to read by the person you are writing to. If you have nice handwriting, definitely go that route as it will look nice for the person you write to. There is nothing wrong with a typed letter as long as the content is good.
Some will reply as quick as 2 to 3 days, some will take many years to reply. That's life in this hobby, but generally a good number will reply within weeks or months. It can differ from one requester to another based on luck and timing.
Research is also key. Before writing, do some homework. Is he person you are writing to ill or unable to reply, etc. Double check any addresses you may see listed on autograph websites. Some may be old or incorrect.
Always send your own item(s) for signing. Do not expect something for nothing in this hobby. The people who do this may get successes, but over time it takes it's toll on the hobby, and whether some celebs sign or not. Obvious exceptions are shows like EastEnders where they offer to send their own pics.
Once the replies start coming in you will get hooked. Have fun and enjoy. I have been doing this for 12 years now with over 2000 successful replies, and still enjoy writing the letters, and seeing what turns up in the post each week. It can bring some genuine surprises.
