Boron(B) is a micro element which is also immobile. It is needed for root and bud development while also keeping the stems and branches strong.
Being an immobile element means the deficiencies will show up on the younger leaves first by turning the leaves yellow. It can also show up as abnormal growth tips. Boron(B) will also be absorbed poorly by your plant if there is low Potassium(K) content.
Boron(B) can be locked out of your plant in a soil medium if the PH is lower than 5 and higher than 7.7. In hydro and soilless mediums it is locked out lower than 5 and higher than 6.
Once you spot a Boron(B) deficiency you can either water or foliar spray one teaspoon of Boric acid (sold as eyewash). You can also use bone meal or Borax as well.
In San Luis Obispo at the Healthy Harvest store they recommend getting one of their fine micro-nutrient products which have small amounts of all the micros your plant could need including Sulfur(S). In particular they like the CNS17 from Botanicare. It works out real nice that as long as your plants aren't flowering it works as an excellent foliar feed so the results will show up much quicker. You can also use this as an additive to your water which works good too.
At Garden All Year Nick says he likes natural and organic solutions like compost teas with worm castings and General Hydroponics forest humus. He also really goes for using Azomite as it's works great on all micro nutrient deficiencies as well as it can lower your soils PH if you have problems in that department too.