A reasonably simple change to leveling provides a better opportunity for characters to make incremental progress between levels.
The mechanism is fairly simple.
1. The player writes down a list of all upward adjustments to the character's scores, abilities, skills, etc. which would be gained at the next level. Each item on the list should contain one 'atomic' adjustment, such as "base attack bonus +1" or "will save +1".
2. The GM may stipulate that certain adjustments may only be achieved when the character's level officially increases (when XP required for normal levelling is earned).
3. The player determines the total amount of XP required to progress between the start of her character's present level and the start of the next higher level. This amount is then divided by the number of items (plus one) on the approved list of inter-level adjustments, thereby creating a number of inter-level XP milestones.
4. As these inter-level XP milestones are achieved the player may select an item from the inter-level adjustment list and adjust his character's sheet accordingly.
5. The GM may recommend that a particular inter-level adjustment be made at any given inter-level milestone, based on the relevance of that adjustement to the actions of the character in-game. (For example, if the character rolled a particularly meaningful saving throw vs. poison during the previous encounter, and the milestone "fort save +1" appeared on the inter-level list, it should be obvious to both the GM and player that the players fort save should advance next.) The decision about which inter-level adjustment to apply should be left to player, if no critical impact on the game is at stake. The GM does have the absolute final say on which adjustment must be taken at any inter-level XP milestone. The player should respect the GMs recommendation when given.
6. Individual players are not required to use the inter-level system. Some players may opt to use the inter-level system and other may simply use the normal leveling system. The effect on game balance and relative character effectivness should not be significally compromised.
7. For the sake of game momentum it is recommened that the GM not use the inter-level system for NPCs. The GM has a plethora of standard 'game-mastery' options to equalize game-balance should it seem tilted too much toward the player's advantage. It is unlikely this tilt is significantly determined by inter-level adjustments.
8. The purpose of introducing inter-level milestones is to give the players a sense that their character is actively achieving the next level, progressively acquiring new advantages over time, rather than everything changing all at once.
9. Inter-level milestones give players a chance to become familiar with new abilities little-by-little. Instead of the feeling of playing a new higher-level character when standard XP level is achieved, the character can develop into the higher-level character.
10. As noted, some advancements may only be properly obtained, or make sense, when the character achieves the full XP for standard level advancement. In all cases, the GM should assure that some significant level benefit or benefits are awarded only when full XP is achieved for the character's next higher level. The attainment of XP for the actual next higher level should still be significant milestone, with an accompaning sense of real achievement and benefit.
It is the player's responsibility to keep track of inter-level progress. Once the GM authorizes an inter-level schedule the player must manage any changes to the character's abilities and performance as each inter-level XP milestone has been achieved.
The player may opt to not apply inter-level advantages as rapidly as the inter-level schedule permits. This is entirely the player's choice; it does not affect game-play. Once the player does decide to apply an approved inter-level adjustment to the character, this change should be considered irreversible.
If the player loses a level (gains a negative level) at any point, for simplicity, the GM may opt to nullify the inter-level agreement and require the player to re-acquire levels and acquire the next higher level using the standard leveling system.
Again, the purpose of an inter-leveling system is to provide an enriched experience for players, to give them a more tangible sense of character development and progress than is provided by the standard '3.75' leveling system.
Example 1:
Ardamir, L4 Human Wizard
List of L5 benefits:
caster level +1
3rd level spell
bonus feat
The GM rules that "caster level +1" and the "3rd level spell" should be gained only on full completion of L5 XP requirement. The GM allows "bonus feat" to be placed on the character's inter-level milestone list.
Using Medium XP leveling, the difference between L4 XP (9,000) and L5 XP (15,000) is 6,000. Therefore, at XP 12,000 Ardamir gains his L5 bonus feat.
Example 2:
Vecha, L10 Human Fighter
List of L11 benefits:
bonus feat
first attack +11
second attack +6
third attack +1
armor training 3
The GM rules that "third attack +1" should be gained only at full attainment of L11 XP.
Using Medium XP leveling, the difference between L10 XP (105,000) and L11 XP (155,000) is 50,000. Therefore, Vecha has four inter-level milestones at 115,000 XP, 125,000 XP, 135,000 XP and 145,000 XP.
When reaching 115,000 XP Vecha opts for a bonus feat. Then during the encounter in which Vecha earns the XP for the next milestone, she lands a devastating critical hit on one of her second attacks, killing a major adversary. Therefore the GM and player agree that Vecha should opt for "second attack +6" as her inter-level benefit at the 125,000 XP inter-level milestone, in recognition of her experience of a noteworthy second attack success.
Note that inter-level milestones may be increasingly interesting to higher level characters both because of larger XP gaps at higher levels and because higher-level characters often gain a somewhat larger number of benefits per normal level advancement.
In summary, an inter-level XP milestone system does not add significant complexity to the game, nor does it affect game balance in a drastic way. It provides players with an opportunity to experience character advancement at a more steady, incremental pace. And it gives players and GMs potential opportunities for characters to advance in ways which are more timely and relevant to events in the game narrative.