Senjutsu

refers to the user's chakra created from spiritual and physical energy within the shinobi as well as energy from nature. This drastically increases the strength of all ninjutsu, genjustu, and taijutsu. It would seem that it stems from the toads, as only Jiraiya, Shima, Fukasaku and Naruto can use it. Gamakichi also seems to be familiar with it.

Requirements for using Senjutsu
Only those who possess a high chakra level can invoke Senjutsu. The user is required to balance his physical and spiritual energy with the natural energy he gathers from Nature. If the user puts too little natural energy into using Sage techniques, the technique will not initiate. If the user puts too much natural energy into the techniques, the user will start to turn into a frog. Furthermore, while gathering Natural Energy, the gatherer must remain absolutely still in order to be one with Nature.

Method of Reversing Toad Transformation
Getting hit by a special scepter during the transformation will reverse the Toad Transformation by knocking all the Natural Energy out of the Gatherer. It is dangerous to learn how to gather Natural Energy without the staff because there is no other known way to reverse the toad transformation. Furthermore, if the Transformation is completed, not even the staff can reverse it.

Further Information
Jiraiya, one of the legendary Sannin, had trouble using his sage form. He slightly turns into a toad whenever he uses his sage form. This involved his nose and eyes(altough the eyes can be excluded because as Fukasaku said the "toad eyes" are always present in hermit mode being the sign of a true hermit) changing to a more toad-like appearance. Also, he grows a goatee. However, Naruto, who has just learned how to sense natural energy and create sage chakra, only has red rings around his eyes and horizontal, slit-pupils, showing his mastery over his Sage form. He is able to use his sage chakra very well. He was able to lift a giant frog statue with his sage chakra. However, he accidentally drops it. Fukasaku then said that even though Naruto is learning at a faster pace than Jiraiya, he's still very clumsy.

Before Naruto was able to utilize the sage chakra correctly and evenly, he used frog oil, which allowed the natural energy to enter his body.