Need More Mana

I have a lvl 20 Me/Ele and she only has 40 points of mana. I find that when i am in the midst of fighting i run out of mana quite frequentley, even when i cast spells that require 5 or 10 points. If someone could please tell me how i could fix this..it would be greatley appresheated.

Energy Tap, Energy Drain, Power Drain and Drain Enchantment are your friend
There are some more that give you energy (Inspired Hex/Encantment etc) but Power Drain is by far my favorite. A cheap interrupt and you gain energy with it.

But, 40 energy is rather low.
I have 50 energy on my mesmer, +10 from a Staff (standard on a max dmg Staff in most cases), +5 from a Insightfull Staff Head and +5 from my armor (I own a 15k Kurzick armor - I have a chest and a hose part that give +3 and +2 energy.)

So try adjusting your skillbar a bit and get some new gear with it.

You should try Enchanters armour, and possibly even use a Rod/+5 weapon and Focus instead of a staff for more actual armour. Now to stop you burning out mid battle, the entire Inspiration Magic attribute is related to energy. Fast Casting is inefficient above 7 levels in it, so try some inspiration spells (such as energy tap, channeling, power drain, or the elite mantra of recall.. the list goes on)

Well...your second profession tells me that you're using some elemental spells. I do not know what your spells are, but I recommend Elemental Attunement and [water/fire/air/earth] attunement. I don't think energy tap or drain will give you enough energy for your elementalist spells. You might want to get a +5 and +30 Katana sword with a focus item. This combo gives you the most energy and health. I hope this helps.

Be aware of any elementalist skills you are using that may cause exhaustion. The mesmer professions has a plethera of skills available to ease energy management, most of the better ones already mentioned. The ele class, however, also has those. Play around with attunements if you are utilizing the ele skills, or incorporate glyphs into your build. Alot of times, it's just a trial and error process.

Thanks all, the feedback was very helpful indeed. =)