My Infill (the cross pattern inside my printed object) is a bit weak

Your infill should more-often-than-not not be weak, and the biggest reason for this would be an overall failure to extrude properly; the necessary steps should be taken from the other help sections on how to clean out your extruder or you should make sure that your settings and amount extruded are proper for the nozzle diameter and speed which you are printing at.

If you want a sturdier Infill setting, check out the CraftWare section “Infill” located under the “Basic” tab after you click on “Slice”.  At this point you can chose your pattern, percentage and the thickness of each line, plus a few other more in-depth tweaks.