The first key to a great holiday is planning. The more you plan before you go the more flexible you can be while on holiday. Planning can be about simple things such as adding a space blanket or rain coat. Also it can be as complex as setting up business meetings, finding a publisher for your book while on holiday.
Personal Experience
Nothing is as good as your own experience but do not let this stop you from seeing new places. Do not go back to the same hotel, the same beach. You will miss out on interesting places. Take an afternoon and go and have a look at other places so that you know what is available.
Friends
Use the recommendations from friends but also keep in mind what they prefer. They might enjoy camping and you do not. Respect your own gut feeling and when in doubt, rather go somewhere else.
Tourist Offices
In big cities you might have a tourist office that can supply you with information and brochures about the places you want to visit. Other tourist offices will help you when you enter the city. They can be excellent but in some places they are useless. Do not rely on them alone to get your information.
The Internet
This medium became the first place you should consult. The internet is filled with pages about personal experiences and adverts by the hotels and guesthouses themselves. Some blogs will warn you about a place. Always read with an open mind as they can also be rude and wrong.
Travel Agents
This used to be the place you would go but I think they are for lazy people. What is wrong with you that you can not do this yourself? Searching became easier and it is fun. Search Engines have replaced the travel agents and they can work out cheaper. Remember you get good and bad agents. Some will sell you the bag on which they get the most commissions. Shop around and learn to ask the hard questions.
Newsletters
Again you get excellent, fair and bad newsletters. Use them for money saving tips and the latest travel offers. You can get free ones and others that is expensive. While $40 annually might not be that much it depends on how much you need it and how specific it focuses on.
While there are other ways in finding your information such as in flight magazines and newspapers, you will have to use at least one of the above and maybe more to get what you want. Rather pay a few dollars for good advice than spending thousands because of bad advice.