How to place a layered Photoshop PSD in a page layout
Layout by Zaweba places layered PSD and PSB files and lets you choose which picture layers and layer comps show, without flattening the file first.

Place the .psd file in Layout by Zaweba as you would any picture, and it keeps its layers. You can then choose which picture layers are visible and pick one of the file's layer comps, so the same PSD can show different versions on different pages without exporting a separate flattened copy for each. Large PSB files work the same way.
Designers often build pictures in Photoshop with variations in layers: a product with and without a shadow, a photo with alternative backgrounds, a label in several languages. Placing the layered file directly keeps those choices open right up to the end of the job.
Placing the file
Draw or select a frame and place the PSD. Like other pictures, it can be linked or embedded. Linked is usually better for layered files: the PSD stays on disk, the layout points to it, and if the file is edited in Photoshop the Links panel shows it as modified so you can update. Embedded copies the picture into the document, which is convenient for a self-contained file but makes the document bigger and cuts the link with the original.
If you have several pictures to place, the place gun lets you load many at once and drop them into frames one after another, with thumbnails to show which is next.
Choosing layers and layer comps
Once placed, pick which picture layers show. Turn off a background layer to let the page show through, or switch on an alternative version of a product shot. Layer comps are saved combinations of layer settings made in Photoshop. If the designer of the PSD has set them up, choosing a comp is the quickest way to switch between agreed versions, such as summer and winter variants of a catalogue cover.
Keeping one layered source file for a whole family of images means fewer exports to manage and fewer chances of a version going astray.
Fitting and cropping
The frame and the picture inside it are separate. Use the fitting options to fill the frame proportionally, fit the whole picture in, or fit the frame to the picture, then adjust the crop. Clipping paths can come from the picture's alpha channel, which is handy when a PSD has a transparent background and you want text to wrap around the subject. Text wrap can follow the alpha channel too.
Checking quality before print
The Links panel shows each picture's status, colour space, size, and its effective and actual ppi. Effective ppi is what matters for print, because it takes the scaling in the layout into account. A PSD that was fine at full size may be too low resolution once enlarged across a spread.
Live preflight helps here as well. It can flag low-resolution pictures and RGB pictures in a print job, which matters because many PSDs are kept in RGB. Whether to convert to CMYK in Photoshop or let colour management handle it at output is a question for your printer and your colour settings.
Good habits with layered files
Keep layer names clear in Photoshop, because they are what you will see when choosing which to show. Keep the PSDs together in a folder next to the layout so links don't break when the job moves. When the job is finished, package it: the package collects the document, linked pictures and fonts where licences allow, so the printer or the next designer gets everything in one folder.
Placing the layered file rather than a flattened export means fewer copies to manage and fewer chances of putting last week's version in this week's print run.