Expert dealers who you can trust are less common than the rarest of antiques and arguably more valuable.
If you do find one, look after them and don’t forget that relationships need to be mutually beneficial. You should not expect dealers to share all of their trade secrets and try not to be put off by knowledge of their mark up. They have to buy and sell to earn their living and will be up early hunting for pieces which you might otherwise never see.
The guru who befriended me has been collecting in this area for nearly 40 years. At times I have felt that extracting the pearls of wisdom needed a good vantage point and a jemmy, but in truth this generous man has given me an education which could not have been found elsewhere.
Another great resource is through like minded collectors where a common enthusiasm can become a good basis for sharing knowledge. Here too there can be conflict of interest in finding and securing mutually desirable antiques but this does not have to become an obstacle. If dealt with on a one to one basis a system of taking turns works well enough and there always seem to be enough antiques to go around.
Read up on your specific area. Find reference books, articles and catalogues which are relevant to your collection. The latter is especially important as pieces which can be identified from a catalogue will be considered to have a provenance that unidentifiable pieces do not and this will increase their value.
Museums are great sources for research and documentation, but do not assume that their records are written in stone. There may be some specific areas of interest where you have done your own research where you exceed their knowledge and become an expert in your own right. Some years ago an interested curator would be considered as an invaluable source of information. As the financial constraints on museums become further pressured it has become the norm to find curators who are more orientated towards business skills and accountancy than history and academia. But if increasingly hard to find, they are still worth seeking out.