How to reduce file size in Revit?

Save it as another file, this sometimes works.

Stop linking CAD files into your Revit models!!!!!!
Delete all unused image files, you can delete, but they stay in the model and new instances will be created if you import more images, use 'manage images' to remove them completely

If it is anything like the files I get from "Revit expert" architects will bet money it has -
1- It has 10000000 unused/old views from every man and his dog in it,
2- Stupid undisciplined families from every man and his dog and horse that they found on the internet ages ago. (a bad family can hugely bloat a model size, the problem is usually when they upgrade from previous revisions)
3- Unpurged

I'm at 7Gb for our model and BIM manager has gone off ill. decentralisation has reduced to a mere 3Gb there are far too many families in this model plus a lot of editing history in each family. its killing the model...

1. Purge model
2. Delete unused famililes.
3. Delete reference line & detail line, model line which is not in used in family & same as follow for project.
4. When you linked or import cad file in model then file size will increase, delete or removed cad file once you finish your work.
5. When you save central file, tick on compact file option then file size will reduce.

It's hard to answer the question without looking at the model, but here's few basics you can try -
1. Purge the model to remove unused families, materials, etc.
2. Try to look at used families' sizes. Take the biggest ones and purge them or even simplify them for your needs - ad load them back into model.
3. Get rid of any unnecessary imported or linked CAD files. While linked files are easy to remove from dialog box, imported files are a nightmare. But there are a few add-ins for this. Search 'reviTTools' on Google.
4. See if you can delete any redundant views or sheets.
5. If you have any 'In-Place' families, try to replace them with component (external) families.
6. At last, detach-audit the model.

If all this does not get you a fairly workable model, you might want to split the model into multiple Revit files - e.g. one for exterior, the other for interior etc.

Purge also all the families that you have in the model.

I once came across a project file that was 330 Mb (2-Story 15,000 s.f. Public Library Renovation with a 12,000 s.f. addition). As I investigated it, there was a simple profile family being used in a fascia that had a .tif file linked in. Removing the .tif file and re-loading the family brought the project down to 47 Mb! I concur with all previous suggestions. CAD links and in-place families are typical culprits of excessive file size.

Compact your file, good luck!

Purge it.