I think some people suggest to roughly ESTIMATE what the answer might be, before starting the actual calculations. Then, if the calculations produce a VERY different result, there is probably an error somewhere.
And, as one would do with writing, to PROOF-READ the work, before deciding it is complete.
Maybe design some kind of 'check list' that is appropriate to each type of problem normally encountered; her teacher may be able to help with this, and might recommend the idea to other pupils.
Read the question TWICE to make sure it is understood; it may take a bit longer, but could possibly reduce careless mistakes.
I give below my standard Numeracy advice, which is aimed at much younger children, but it may clarify weaknesses or help to clear up confusions:
Practical things are best for grasping number concepts - bricks, Lego, beads, counters, money, shapes, weights, measuring, cooking.
Do adding, taking away, multiplication (repeated addition), division (sharing), using REAL OBJECTS as just 'numbers' can be too abstract for some children.
Number Bonds of Ten forms the basis of much maths, so try to learn them. Using Lego or something similar, use a LOT of bricks (of just TWO colours, if you have enough) lay them out so the pattern can be seen of one colour INCREASING while the other colour DECREASES. Lay them down, or build up like steps.
So:
ten of one colour none of other
nine of one colour one of other
eight of one colour two of other
seven of one colour three of other
etc,
then of course, the sides are equal at 5 and 5; after which the colours 'swap over' as to increasing/decreasing.
To learn TABLES, do them in groups that have a relationship, thus:
x2, x4, x8
x3, x6, x12
5 and 10 are easy
7 and 9 are rather harder.
Starting with TWO times TABLE, I always say: "Imagine the class is lining up in pairs; each child will have a partner, if there is an EVEN number in the class. If one child is left without a partner, then the number is ODD, because an odd one is left out."
Use Lego bricks again, lay them out in a column of 2 wide to learn 2x table. Go half way down the column, and move half the bricks up, so that now the column is 4 bricks wide. That gives the start of 4x table.
Then do similar things with 3x and 6x.
With 5x, try and count in 'fives', and notice the relationship with 'ten' - they will alternate, ending in 5 then 10.
It is important to try and UNDERSTAND the relationships between numbers, and not just learn them 'by rote'.
An inexpensive solar powered calculator (no battery to run out!) can help learn tables by 'repeated addition'. So: enter 2+2 and press = to give 4. KEEP PRESSING = and it should add on 2 each time, giving 2 times table.
There are good web sites, which can be fun to use :
www.ictgames.com/
www.woodlands-junior.kent.sch.uk/page/default.asp?title=Woodlands%20Junior%20School&pid=1