Make an appointment - most archives are held out of public view in dark basements and staff need time to retrieve documents on your time on the day will be wasted.
If you can, obtain a catalog of items available and pre-request the ones you want to start with.
Bundles of documents are often indexed hierachically. For example archive SBX123 will contain documents like SBX123/70 and that in term may have sub-documents SBX123/70/1/1-8 This is important because archives often limit how many documents you can pre-order and view at one time. The archivist may allow you all of SBX123/70 if it contains lots of small parts, but you might be disappointed if you just ask for SBX123/70/1/5 and it turns out to be a single piece of paper.
Take a camera. Most archives will charge either for photocopying, cameras or both but by taking copies you can continue to enjoy what you've found once you get back home.
A tripod is useful too to steady your camera/phone and get good quality images.
When photographing images, if you have one, a non reflective A4 glass pane would have been very useful for keep images flat without also damaging them.
Magnifying glasses are often provided but very useful if you have a favourite.
Pencils are often provided, because pens are not allowed.