A demo is a way for musicians to approximate their ideas on tape or disc, and provide an example of those ideas to record labels, producers or other artists.
Unfortunately, there are no sure-fire ways of to avoid having record company execs toss demos into a pile of trash with all others they are not interested in.
One thing you could do, though, that would improve your chances is to find an agent. These people are usually better connected and have a better chance of getting your demo listened to by the right people.
Large bookstores usually carry a yearly guide of who does what in the business. Make sure you get the most recent edition, lots of people move or change places. People get fired and resigned. Execs don't like receiving mail addressed to their predecessors.