One highly effective method of increasing reach in Pathfinder, not yet mentioned, is the Cleric Subdomain Plant(Growth). This option is different from, and superior to, many other methods of increasing reach in Pathfinder. This domain option provides more than just Swift Action Enlarge Person. It also provides the character with tactically valuable on-demand extra 5' steps.
Enlarge (Su): As a swift action you can enlarge yourself for 1 round,
as if you were the target of the enlarge person spell. You can use
this ability a number of times per day equal to 3 + your Wisdom
Replacement Domain Spells: 1st—
Using the enlarge ability also provides the user with free virtual 5' steps!
I've played a Plant(Growth) Cleric in Pathfinder through 10 levels and can testify that it is extremely effective. In terms of reach, large size stacks with longarm. A Large biped wielding a reach weapon and buffed with longarm will have 25' reach and a ridiculous 60' diameter threat circle. The enlarge ability allows one to walk around normal-size yet swiftly become large if combat occurs.
The best part is the free virtual 5' step that comes from frequent starting and stopping of enlarge person. This is a tactical move that will make you smile every time you use it.
Getting Large: Your unbuffed longspear-armed PC can threaten 30' away with just a 5' step. Take a 5' step toward the foe, then swift Action enlarge person and choose to grow toward the foe. You now threaten the foe 20' away and still have your normal actions. It's as if you got an extra 5' step.
Getting Medium: Foes will quite reasonably feel threatened by the spear-wielding giant and may try to run in close en masse. Say foes survive your Attacks of Opportunity and move adjacent to you. When your turn comes around the first thing that happens is that enlarge person ends. Choose to shrink away from your attackers. They are no longer adjacent. It's as if you got an extra 5' step.
You can even 5' step away and swift enlarge person again, choosing to grow away, which leaves large you threatening foes who are now 15' away. You may full attack or safely cast a spell, yet foes will draw more AoOs if they want to close with you again. This works exceptionally well for area denial builds using a reach weapon. You can prevent foes from landing a Full Attack, yet you can always Full Attack.