A88 complement
As pointed out, voluntary XP sacrifices for casting spells or creating magical items cannot bring you below the minimum XP score for your level. You need to have enough XP free to sacrifice before starting either or the activity fails.
The only way (that I know) of losing XP that can reduce your current level is the XP reduction following the loss of a familiar, item familiar or psicrystal; those can drop you a level if you're reduced below the current minimum.
A89 complement
In addition the aforementioned methods, there are also the Bloodlines rules from Unearthed Arcana. Each bloodline level (up to 3 with a major bloodline) raise all of your maximum skill ranks by 1 without changing your character level.