When stretching for the purpose of increasing overall flexibility, a stretching routine should accomplish, at the very least, two goals:
1. To train your stretch receptors to become accustomed to greater muscle length (see section Proprioceptors).
2. To reduce the resistance of connective tissues to muscle elongation (see section How Connective Tissue Affects Flexibility).
If you are attempting to increase active flexibility (see section Types of Flexibility), you will also want to strengthen the muscles responsible for holding the stretched limbs in their extended positions.
Before composing a particular stretching routine, you must first decide which types of flexibility you wish to increase (see section Types of Flexibility), and which stretching methods are best for achieving them (see section Types of Stretching). The best way to increase dynamic flexibility is by performing dynamic stretches, supplemented with static stretches. The best way to increase active flexibility is by performing active stretches, supplemented with static stretches. The fastest and most effective way currently known to increase passive flexibility is by performing PNF stretches (see section PNF Stretching).
If you are very serious about increasing overall flexibility, then I recommend religiously adhering to the following guidelines:
Overall, you should expect to increase flexibility gradually. However, If you really commit to doing the above, you should (according to SynerStretch) achieve maximal upper-body flexibility within one month and maximal lower-body flexibility within two months. If you are older or more inflexible than most people, it will take longer than this.
Don't try to increase flexibility too quickly by forcing yourself. Stretch no further than the muscles will go without pain. See section Overstretching.