Here's an alternative for those of you who don't want to keep track of
negative levels, but do want an equivalent mechanic that delays
advancement and can kill.
XP is counted such that each level begins at zero.
If you would receive a negative level, you instead lose 1d6x300 XP
times the source's caster level, or CR if there is no caster level. If
that makes your XP negative, you can't cast spells with XP component
costs or create magic or psionic items, and you take -1 to attack
rolls, skill and ability checks and saving throws. For each 5000
negative XP you have, you get -1 to CON.
Optional rule: If the GM wants characters to be able to build items or
cast spells that cost XP immediately after leveling up, have each item
creation feat and each known spell that costs XP provide an overdraft
limit within which XP can be spent, taking the penalties. Overdraft
limits stack and are as follows:
Brew Potion or Scribe Tattoo: 25
Scribe Scroll or Imprint Stone: 50
Forge Wand or Craft Dorje: 100
Other non-epic item creation feat: 500
Epic item creation feat: 2500
Spells: Total of their XP costs, to a maximum of 100 times caster level