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 and enables the user to enter a powered state called Sage Mode. It would seem that it stems from the toads of Myobokuzan, 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/her physical and spiritual energy with the natural energy he/she gathers from nature. If the user puts too little natural energy into using senjutsu, 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 third of the legendary Sannin, had trouble using Sage Mode, as he slightly turns into a toad whenever he uses this form. This involved his nose and eyes (although the eyes can be excluded because as Fukasaku said the "toad eyes" are always present in Sage Mode being the sign of a true Sage) changing to a more toad-like appearance. Also, he grows a goatee just like Fukasaku's and his fingers are toad/frog-like.

However, Naruto, who has just learned how to sense natural energy and create sage chakra, only has red rings around his eyes and horizontal, golden slitted-pupils, showing his mastery over his Sage form--Naruto's mastery of Sage Mode may be owed to prior experiences in "Jinchūriki Mode". He is able to use his Sage chakra very well. He was able to lift a giant frog statue with his Sage chakra augmenting his physical strength.

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. However, the oil involved can only exist in liquid form in Myobokuzan's atmosphere, and evaporates if it is taken outside, thus forcing Naruto to gather natural energy on his own. Jiraiya was able to continuously draw in natural energy by having Fukasaku and Shima remain motionless on his shoulder while he fought, but the Nine-Tailed Demon Fox rendered this impossible for Naruto.

In order to work around this problem, Naruto had two of his clones gather natural energy, and dispelled one when he ran out, gaining the clones' Sage chakra. Furthermore, usage of high-level techniques while in Sage Mode can hasten the exhaustion of the user's Sage chakra reserves, as was seen when Naruto quickly reverted to normal after he fired a Wind Release: Rasenshuriken at Pain's Human path and a Sage Technique: Rasenrengan at Pain's Animal path.