When it comes to volunteers to the army we need to consider three types.
1. Machal Volunteers - Overseas volunteers who do not take out citizenship
2. Extra Service - People who volunteer to lengthen their service so that they can get into certain positions
3. Service Volunteer - People who volunteer to serve in the army even though it is not required by them.
4. Female Volunteers
Machal volunteers started off as the name for overseas volunteers who arrived to fight for Israel during the war of Independance. Over the years this has developed into an organization which allows any male between the ages of 18 - 23 or women between the ages of 18 - 20 with at least one Jewish grandparent to come and volunteer in the Israeli army without ever becoming a citizen of the state(Note: Doctors can volunteer up to the age of 35). Most of the Machal volunteers will serve in the Nahal Brigade for a period of 14 to 18 months. If at the end of your volunteer program you decide to make aliyah your time in Machal will be thought as part of your mandatory service. For more information please see the Machal site at http://www.mahal-idf-volunteers.org/
Many Olim will be given the oppurtunity to volunteer for extra service and for certain jobs in the army you will be required to sign for extra service. If you are only required to do 6 months of service, your job options in the army are limited and are most certainly not the most glorious of jobs. Not to say they are unimportant jobs, everything in the army is important. You may not feel satisfied out of your service if you do not serve in what is considered a challenging non-combat position or in a combat unit itself. If you would like to serve in a relatively good non-combat spot you will be asked to sign for at least a year and if you would like to serve in a combat unit you will most likely be asked to volunteer for at least 18 months. In other cases a soldier who is meant to serve 18 - 24 months may be asked to sign on more time if he chooses to serve in certain units. In such cases you will usually need to sign for at least 2.5 - 3 years. In some very special units you maybe asked to sign on for more then the three year regular service period. In such cases you will receive a regular salary(known as Keva) following the three years mandatory service.
In some cases you may not be required to serve but you want to volunteer to serve. Understand that their is no guarantee that the army will allow you to serve. What is important that if you are serious about this you will need to consider the consequences. You will be much older then your average Israeli serving in the army, you will have a very low salary, you will not be home very often, if you are currently in a relationship it will most likely become very strained during your service especially if you are not home often, if you are not in one you will have a hard time starting one well you are in the army and many aspects of your life will have to be put on hold. On the other side of the coin you need to ask yourself how will you feel when all your new found friends are being called up to reserve duty during a conflict with one of our many non-friendly neighbours and you are at home virtually helpless.
In the event you do decide to volunteer, you need to understand that it will be very hard for you to convince the army to take you, and there is a chance they may refuse to. If you are serious about it, you are going to have to go to the lishkat giyus(draft office) and tell them you want to. They will most likely tell you that you are too old and that they won't take you. You will then need to go and tell them again, and again, and again until someone starts listening to you. Ask your friends who may have been officers in the army. Write letters. The most important thing is that you don't take no for an answer. There have been rare cases in which people in their mid 30s have managed to volunteer for the army. You need to be persistent, let them know that you understand the consequences and to be very very insistent. I make no guarantees that this will work,but it will certainly increase your chances.
The general rule is that a female can volunteer for the army if she is between the age of 21-22 for a period of 1 - 2 years. The rules above apply the same for females as they do for men.
Volunteering for the army be it for extra time, for service or as an overseas volunteer is a very tough choice, and it will change life. As someone who volunteered to do three years instead of two so that he could serve in a better unit, I can say I have no regrets, it was the experience of a lifetime even if it was at many times a very very hard experience. Don't expect the army to be fun, but take it as an experience and you can get a lot of out the army and donate a lot to the country at the same time.