To boost any individual item, you need to include the following in its inventory metadata (see Defining your Inventory for more information on how to add to inventory metadata):
"value" : "high"
In particular, you can put this information in the LiftIgniter JSON object if sending inventory through JavaScript, or as one of the fields in the inventory items sent via API if you are using the inventory API.
Tweaking overall and per-item boosts
If the standard strength of boost provided by including this metadata is not adequate for your use case, please get in touch with Support to discuss more advanced boosting rules and logic. We can both change the default boost strength and set up rules that allow you to specify different boost strengths on different items. Setting up the rules correctly requires some site-specific analysis, and hence requires getting in touch with Support.