What are the some key factors which needs to be kept in mind when suggesting a proposal?

You have to convince stake holders your proposal benefits the greater good of the hive community as much or more than you are asking. At least that is what it is in theory.

Have you ever created any proposal? If yes, How many of them were approved & funded?

I have created a proposal, and it was funded for only a short amount of time.

I hear that only after all of the big whales approve, proposal gets funded.

Why would you think it is any different, it isn't the smallest stake holder who holds the deciding vote. You either need whales, or lots of smaller votes, it's simple math, not a conspiracy.

If so, How do I get attention of the big whales? I don't want to tag them & get blacklisted.

Do something they care about.

I am already posting dev-updates without any expectations so far for straight 37 days & 3 months in total. But without proposal, I am going to dry out soon.

No one owes you anything, no one asked you to do anything. Keep that in mind when you work on it.