Masturbation (for both men and women) is haraam (forbidden) in Islam based on the following evidence:
First from the Qur'aan:
Imam Shafi'i stated that masturbation is forbidden based on the following verses from the Qur'aan (interpretation of the meaning):
"And those who guard their chastity (i.e. private parts, from illegal sexual acts). Except from their wives or (the captives and slaves) that their right hands possess, - for them, they are free from blame. But whoever seeks beyond that, then those are the transgressors." 23.5-7 Here the verses are clear in forbidding all illegal sexual acts (including masturbation) except for the wives or that their right hand possess. And whoever seeks beyond that is the transgressor.
"And let those who find not the financial means for marriage keep themselves chaste, until Allah enriches them of His bounty." 24.33. This verse also clearly orders whoever does not have the financial means to marry to keep himself chaste and be patient in facing temptations (including masturbation) until Allah enriches them of His bounty.
Secondly, from the Sunnah of the Prophet (peace be upon him):
Abdullaah ibn Mas'ood said, "We were with the Prophet while we were young and had no wealth whatsoever. So Allah's Messenger said, "O young people! Whoever among you can marry, should marry, because it helps him lower his gaze and guard his modesty (i.e. his private parts from committing illegal sexual intercourse etc.), and whoever is not able to marry, should fast, as fasting diminishes his sexual power." Bukhari:5066. The hadeeth orders men who are not able to marry to fast despite the hardship encountered in doing so, and not to masturbate despite the ease with which it can be done.
There are additional evidences that can be cited to support this ruling on masturbation, but due to the limited space we will not go through them here. Allah knows what is best and most correct.
As for curing the habit of masturbation, we recommend the following suggestions:
1) The motive to seek a cure for this problem should be solely following Allah's orders and fearing His punishment.
2) A permanent and quick cure from this problem lies in marriage as soon as the person is able, as shown in the Prophet's hadeeth.
3) Keeping oneself busy with what is good for this world and the hereafter is essential in breaking this habit before it becomes second nature after which it is very difficult to rid oneself of it.
4) Lowering the gaze (from looking at forbidden things such as pictures, movies etc.) will help suppress the desire before it leads one to commit the haraam (forbidden). Allah orders men and women to lower their gaze as shown in the following two verses and in the Prophet's hadeeth (interpretations of the meanings):
"Tell the believing men to lower their gaze (from looking at forbidden things) and protect their private parts (from illegal sexual acts, etc.). That is purer for them. Verily, Allah is all-aware of what they do. And tell the believing women to lower their gaze (from looking at forbidden things) and protect their private parts (from illegal sexual acts, etc.)... " 24.30-31
Allah's messenger said: "Do not follow a casual (unintentional) look (at forbidden things) with another look." Al-Tirmidhi 2777. This is a general instruction by the Prophet to abstain from all that may sexually excite a person because it might lead him/her to commit the haraam (forbidden).
5) Using one's available leisure time in worshipping Allah and increasing religious knowledge.
6) Being cautious not to develop any of the medical symptoms that may result from masturbation such as weak eyesight, weak nervous system, and/or back pain. More importantly, feeling of guilt and anxiety that can be complicated by missing obligatory prayers because of the need to shower (ghusl) after every incidence of masturbation.
7) Avoiding the illusion that some youth have that masturbation is permissible because it prevents them from committing illegal sexual acts such as fornication or even homosexuality.
8) Strengthening one's willpower and avoiding spending time alone as recommended by the Prophet when he said "Do not spend the night alone" Ahmad 6919.
9) Following the Prophet's aforementioned hadeeth and fast when possible because fasting will temper one's sexual desire and keep it under control. However, one should not overreact and swear by Allah not to return to the act because if one does not honor one's promise, one would be facing the consequences of not living up to one's oath to Allah. Also, note that medication to diminish one's sexual desire is strictly prohibited because it might permanently affect one's sexual ability.
10) Trying to follow the Prophet's recommendation concerning the etiquette of getting ready for bed, such as reading well-known supplications, sleeping on the right side, and avoiding sleeping on the belly (the Prophet forbade sleeping on the belly).
11) Striving hard to be patient and chaste, because persistence will eventually, Allah willing, lead to attaining those qualities as second nature, as the Prophet explains in the following hadeeth: "Whoever seeks chastity Allah will make him chaste, and whoever seeks help from none but Allah, He will help him, and whoever is patient He will make it easy for him, and no one has ever been given anything better than patience." Bukhari:1469.
12) Repenting, asking forgiveness from Allah, doing good deeds, and not losing hope and feeling despair are all prerequisites to curing this problem. Note that losing hope is one of the major sins punishable by Allah.
13) Finally, Allah is the Most Merciful and He always responds to whoever calls on Him. So, asking for Allah's forgiveness will be accepted, by His will.
And Allah knows what is best and most correct