It's a simple principle really, but often ignored. The idea is to order your life according to what God says is important. If you have never looked at what God's priorities for you are I encourage you to search the scriptures and take that time. It can literally be life changing.
Over the years I have to say the thing that has helped me in decision making and trying to maintain that "balance" is respecting Gods' ordered priorities in my life. Am I always perfect in this, not at all, but I'm pretty good at respecting this and it has kept me in God's will all of the time. That is really important to me.
Another book/study that is really helpful in this area is, Life Management for Busy Women by Elizabeth George. I had been reminded of this book recently as I searched my brain for what gift I wanted to give to a young lady close to us for her graduation and then again as I was talking with my pastors wife last week at a beach outing.
It's actually been a number of years since I've read it but it was so good I believe I've lead three different groups in Bible Study with this tool. From those three studies I saw more positive life change in the women of the groups I was leading than in any other studies I've used (and I have lead quite a few!)
I can't possibly quote all the scripture or go into all the details but the idea is you purpose ahead of time what has priority or takes precedence in your life AND you stick to it! It's pretty clear in scripture that God wants first place and if you are married it's also pretty clear that your husband should be second. From there you'll want to place family, house, ministry, outside work etc... Some of these priorities are clear cut in scripture and others you may have to wrestle with and pray over.
A great place to start looking over God's priorities for Women would be Proverbs 31. Mark 12:30 also gives us our #1 priority clearly, "You shall love the Lord your God with all your heart, with all your soul, with all your mind, and with all your strength. This is the first commandment."
Once you've figured out your order you practice them by using them as a guide for your choices. For instance, you are asked to teach Sunday School. Well you must run that down your priorities. Two questions to ask here: Are your top priorities taken care of? Do you have their permission for taking on something else?
Have you been taking care of the relationships and needs of your top priorities first? If you've been neglecting these there is no reason to pour more time into priorities farther down the list until you take care of your top priorities.
Ask God what he thinks of you taking this on, ask your husband. If either answers no the answer is no.
This is so simple to say, harder to do sometimes and even harder for others to respect but it's Biblical.
I've been trying my best to live out my priorities for many years now. Most of the time I think I do a pretty good job. The harder part for me lately is others not only not understanding but even condemning me for obeying God.
That is REALLY tough! Remembering WHO is most important however, "A woman who fears the Lord, she shall be praised." Proverbs 31:30
So remember that, even if others don't or won't understand your choices if you stay in God's will there is no better place to be!