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I made a hook recently that I put up with VHB tape. It fell off around 2 weeks later (the hook come loose from the tape). It's just a hook for an empty backpack, so let's say it is under medium load.

Every time I design something like this I wonder if it helps to roughen up the surface before applying the tape. I'd imagine that the tape grabs onto grooves made with a knife or sandpaper but I am not sure tbh. I've seen some videos where someone preheats the tape before applying it but without explaining why.

Does anyone know how that works best?

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If you ook into the VHB design document, the section "How to Prepare Specific Surfaces" describes how to treat the surface:

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For higher adhesion, a primer should be used according to the document.

This reference describes that roughing up plastic parts can be beneficial for the adhesion of some tapes:

Abrade the Surface: Roughing up the surface (i.e. sanding) will loosen up any accumulated dirt, rust, or chipped paint. It will aid in the adhesion to painted surfaces or plastic items. A finely- abraded surface with shallow scratches created by a circular motion (rather than straight lines) has the best potential for a strong and persistent bond. This method can create up to a 40 % increase in surface area and can result in greater immediate and long-term bonding potential. Scrub pads, fine steel wool, or sandpaper can achieve the right level of abrasion. A palm sander could be helpful for larger jobs. Avoid using coarse abrasive materials because a too-rough substrate may inhibit the adhesive flow onto the surface. Always clean with the IPA/Water solution, or other solvents, and make sure all loose particles are removed. While it is not typical, some high-bond tapes adhere best to smooth, glassy surfaces, so double check with the manufacturer before you abrade the surface and compromise the bond strength.

Both references describe the use of a cleaning solution based on 50 % water and 50 % IPA.

A higher temperature is beneficial for obtaining the bond strength faster, the VHB design document shows that increasing temperature shortens the time when the full bond strength is reached:

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