Firstly I am not a saddler or a bit/bridle fitter but I wanted to write a blog on this subject after I few things I see frequently online.
Firstly - The saddle 
I have been through tons of saddles with my 2 horses as I have had them since youngsters they are ever changing and one saddle does not fit forever. I have had my saddler come to adjust my saddles just over the changing seasons as they often end up getting a few cheeky extra pounds when they are on grass and I can really tell especially with my 6 y.o as he becomes restricted at the shoulder ( where most his weight goes besides his belly ) 
I feel it is really important for horses to be measured and fitted with the correct saddle as there are so many long term effects from an I'll fitting one. 
Just because it looks ok does not mean it is unless your a saddler.
There are many factors to take into consideration including the horses shape, spine space , clearance at the shoulders , length of saddle and width but it's also very important the saddle fits you. 
Often I see people advertising for saddles online asking for certain inch seats and it needs to be 10inch D to D. This is not accurate each saddle can have different d ring placing. Then there's the old fashioned coat hanger way, this again doesn't include the above factors that have to be correct for the horse. 
I have had horses over the years with bad backs, trauma, muscle wastage and generally traumatised from I'll fitting saddles. A simple visit from the saddler could save this and also expense and damage in the long run. 
I often see when people are looking for saddles they ask for a certain seat size this to is not always the same depending upon the saddle. My gelding take a 16.5 - 17" saddle but he's currently in an 18" seat and 17" saddle. These are all things to consider. 
Thicker Saddlecloths and half pads should be taken into consideration also,  they can look pretty but sometimes they may be making your saddle too tight / perched on top. 
I feel like bridles are not spoken about as much as saddles but these are also a very important factor for your horses comfort and way of going especially as  There are many sensitive nerves in the face.

I have always gone by the rule of the nose noseband  being 2 finger gap away from the cheek bone and I don't fasten it tighter than being able to fit 2 fingers freely underneath. 
There are options of the anatomic bridles and other new designs for horses that may be sensitive to cavesson nose bands. 
Its important for buckles not  pressing on the nerves/sensitive areas, this can cause tension and discomfort. Correct diagrams can be found for this online.
There should be enough clearance around the ears and 2 fingers just fit easily underneath the brow band. 

The bit shouldn't be to high nor low in the mouth that it is clanking on the teeth / horses mouth pulled tight. usually a healthy couple of wrinkles is right and not be touching teeth. 
I highly reccomend a bit fit coming out to check your horse for bit sizing and type as again they are all different some with fleshy mouths/bigger tongues and little room in the mouth, by doing this you can be sure your horse in happy in the mouth and you can get your best from him in the bit best suited to him.
Teeth are another factor of horses being happy when ridden and just a general part of their welfare especially.
 I have mine done twice a year and each time they are ready to be done. My gelding usually tells me as he becomes inconsistent in the contact. 
Thankyou for reading :)