I need Knowledge The Planes to be a class skill...

[Boostaloo]

...but I am a Half-Elf Beguiler. How do you think I can make this happen?

[woodenbandman]

Educated feat from Eberron, Knowledge Devotion feat from Complete Champion, Apprentice feat from (I think) DMG2 or Unearthed Arcana.

[Emong]

Take this feat.

[Sinfire Titan]

There's an Abyssal Heirtor feat in FC1 that gran ts Knowledge the Planes and Spellcraft as class skills.

[JeenLeen]

As half-elf, you qualify for Able Learner (Races of Destiny). If you take a cross-class level with a class that has Knowledge (Planes), this feat effectively makes it a class skill for you regardless of what class you are leveling up.

[Blackfang108]

As half-elf, you qualify for Able Learner (Races of Destiny). If you take a cross-class level with a class that has Knowledge (Planes), this feat effectively makes it a class skill for you regardless of what class you are leveling up.

One problem. Don't you need to be 1st level for Able Learner?

[bosssmiley]

...but I am a Half-Elf Beguiler. How do you think I can make this happen?

Ask your DM (nicely) if you can sub it in for one of your other skills, or if you can just spend points on it. Skills in 3E do nothing level-appropriate anyway, so it'll be about on par with giving someone a free weapon proficiency feat (i.e.: big whoop!)

Why jump through RAW hoops in what is your game?

[Blackjackg]

Bosssmiley is exactly right. That being said,

Take this feat.

The Cosmopolitan feat from FRCS is better if you don't want to multiclass as, in addition to giving you a class skill of your choice, it also grants a +2 in that skill.

[Devils_Advocate]

Why jump through RAW hoops in what is your game?

Because searching for obscure feats and alternate class features can be an interesting exercise.

Alternately, because your DM is reluctant to deviate from published game material. Even though page 110 of the PHB talks about customizing classes and races by doing minor things like swapping out class skills. Presumably, such a DM trusts the judgment of professional game designers more than he trusts his own. The poor, naive fool...

[scsimodem]

You might also go with cosmopolitan from FR. +3 to a skill and it becomes a class skill.

[Biffoniacus_Furiou]

Knowledge Devotion in Complete Champion. It not only gives you a Knowledge skill of your choice as a class skill forever, but you also get bonuses in combat against certain creatures based on your Knowledge checks.

[Frosty]

You know, there's a much better feat out there from Complete Mage or Complate Arcane called "Favored Magic Foe." Choose a type of foe (like Evil Outsider for example) and you gain +1 to penetrate their SR, and +1 to your spell save DCs vs them. You also gain a skill as a class skill depending on the type of foe you chose. If you picked Evil Outsider, you'd get Knowledge (the planes).

[Haven]

You could take levels in Human Paragon; the first level one lets you choose ten skills as class skills, and then lets you choose one of those to always count as a class skill. And the second level and third levels give you spellcasting, a bonus feat, and an ability score boost.

You'd lose a level in spellcasting (maybe two if your DM forces you to hew to the rule that says you have to take a level of half-elf paragon first, but then you can take the second level in half-elf paragon which is pretty decent), which always sucks, but it'd give you a few d8 hit dice and a bonus feat or two.

[Thurbane]

...but I am a Half-Elf Beguiler. How do you think I can make this happen?

Just out of curiosity, are you going for the fiend-blooded PrC?

[Blackjackg]

You know, there's a much better feat out there from Complete Mage or Complate Arcane called "Favored Magic Foe." Choose a type of foe (like Evil Outsider for example) and you gain +1 to penetrate their SR, and +1 to your spell save DCs vs them. You also gain a skill as a class skill depending on the type of foe you chose. If you picked Evil Outsider, you'd get Knowledge (the planes).

It's in Complete Mage, but you're misremembering the feat description. The "associated skill" is the skill in which you must have six ranks to qualify for the feat (if you don't have Favored Enemy). It is not granted as a bonus class skill.

[Frosty]

Really? Then my DM must've house ruled the feat to make it better.

[Chronos]

The Cosmopolitan feat from FRCS is better if you don't want to multiclass as, in addition to giving you a class skill of your choice, it also grants a +2 in that skill.

That's not what I'm seeing...

Prerequisite: Dwarf (Waterdeep), elf (Waterdeep), half-elf (Waterdeep), halfling (Amn), or human (Amn, the Golden Water, the Sword Coast, or Waterdeep).
Benefit: You gain a +2 bonus on Bluff, Gather Information, and Sense Motive checks.
Special: You may select this feat only as a 1st-level character. You may have only one regional feat.

[Blackjackg]

From Forgotten Realms Campaign Setting, page 34:

Choose a nonexclusive skill you do not have as a class skill. You gain a +2 bonus on all checks with that skill, and that skill is always considered a class skill for you.

It's 3.0, and dated by the reference to "nonexclusive" skills, but since there are no exclusive skills in 3.5, it should still work. And yeah, there is a regional requirement, but that's just fluff and could easily be discarded by a rational GM. Doesn't have to be taken at first level, though.

[only1doug]

Knowledge Devotion in Complete Champion. It not only gives you a Knowledge skill of your choice as a class skill forever, but you also get bonuses in combat against certain creatures based on your Knowledge checks.

+1 to this.

if you plan to use increase Knowledge the planes you will be able to give your entire party a bonus to hit based on a skill check.