Here's a silly little exploit that some may find useful.
You need to have dual spec on your warlock, Destruction for one, Demonology for the other.
Summon your Felguard on the Demo spec (or Wrathguard if you have the Grimoire of Supremacy talent).
Swap specs to Destruction. Cast Flames of Xoroth. Instead of the last pet you had with your Destro spec, you'll insta-summon the Fel/Wrathguard.
Normally, the pet will despawn after a few seconds, but if your destro spec has Grimoire of Sacrifice, you can sacrifice the Felguard and get the Charge ability from it. Useful for farming and maybe ganking lowbies, not sure how useful the charge will be.
If anyone else can find a way to keep the pet permanently (or until it dies), help would be appreciated.