What do you keep in mind while branding your PCBs? Design/professional appearance
How you add your logo will depend on your CAD package, but it is usually importing a DXF file to most of the common options.

As to what to keep in mind, you have silkscreen, copper, and soldermask, which you can use to control how your logo contrasts, e.g. for a darker colour you would flood fill with copper underneath it, for a lighter colour you might try and make sure all layers have an exclusion there so you're left with the colour of the PCB. In some cases you might remove the solder mask where you're trying to make a 2-tone pattern without paying for it.

For example, with gold plating and black soldermask, you can have a mix of off-white, black, and gold by varying what element is on what layer.

For placement, make sure you're crossing as few vias as possible: they will look like dark or partially-copper spots; you can hide small, e.g. 0.3 mm, holes under large areas of silkscreen, but for larger ones try and plan around them.

For the DXF, keep the size, thickness, and spacing limitations in mind from the fabricator: any silk line will be somewhat rounded, so sharp corners are difficult, you may need to smooth out certain parts of your logo to make it suit. 

Give a defined area for the fab to place their serial number, ideally under some other component. I try and give them a 40x10 mm square, and ensure I tell them when ordering to use it; this prevents them slapping it whereever it suits them.

Other things unrelated to the logo, but that I like to do for professionalism are: if the back side is not used, include some notes, e.g. pin names or functions; label a few voltage test points, etc. - saves so many phone calls from someone hooking things up wrong.