One of the biggest barriers to a radically frugal lifestyle is self-deception. We've all done it. We conjure up tricks to justify buying things that deep down we know are foolish. Below I've tried to list a number of ways in which people justify spending money on themselves.

1. I've been so good/stressed out lately, I deserve it. Really? Why do you deserve something nice when others have nothing to eat. A lot of hungry people work harder than you and are far more stressed out than you will ever be. Certainly they deserve food more then you deserve a treat. Try to keep this in mind when you are tempted to buy something. The greater goal to help other people ought to trump our desire for instant gratification.
2. This one little purchase won't make a difference. All savings can make a difference. If you save 10 cents on 10 items a day you can provide for an impoverished child's basic needs. If you're saving your money for a down payment on a house these savings will add up to $365 in one year. Who wants to flush $365 down the toilet? It's all worth it.
3. What will other people think of me if I don't buy x,y or z? Really, this shouldn't matter. This is my biggest weakness though. Advertisements are designed to manipulate this fear. Ideally we should all have enough confidence to shop as our conscience dictates and know that people will come to respect us for our convictions. Another form of this anxiety is the fear that people won't like us if we don't treat them. Gift giving expectations cause us to spend money on things that we would rather not buy. Try explaining your convictions or give to charity in the person's name. This works really well for wedding gifts.
4. I don't have enough time or energy.Some convenience items are worth the extra money. A washing machine, for example, can save you hundred's of hours of time over it's lifetime. However, a lot of convenience items are definitely a waste of money, such as pre-made peanut butter and jelly sandwiches. We should evaluate our purchases and decide whether the added cost is really worth the convenience that it gives us. We should also evaluate whether convenience has other hidden costs like possible harm to the environment. Each person's lifestyle is different so we need to be careful and make sure we're honest with ourselves. Our actions may unintentionally affect others, so it's not just us that we could be hurting.
5. I need this or I'm going to be too stressed out to be any good to anybody. This is probably more a woman's excuse. I've heard people use this defense for buying all sorts of things to make their environment nicer. Is it really true that you would be disabled by living in an ugly environment? Do you need pictures on your wall or nice furniture to be happy? No. Of course this may be a really big sacrifice for you but certainly more joy can be found in helping others. Buying things to make yourself happy will never work. There will always be a need for more.

special Christian deception:
6. I can't reach out to others unless we have a, b or c or spend money on x, y or z. This excuse is valid under very few circumstances. Most of the time, though, this is just an excuse to spend money freely. Also, this is harmful to your witness, because how can you explain to a non-Christian that they need to give up everything when it doesn’t appear that you’ve done that yourself? We can’t live like the rest of the world if we want to help invoke positive change. Some people (mis)use 1 Cor 9:20-22 which is as follows:

   To the Jews I became like a Jew, to win the Jews. To those under the law I became like one under the law (though I myself am not under the law), so as to win those under the law. To those not having the law I became like one not having the law (though I am not free from God's law but am under Christ's law), so as to win those not having the law. To the weak I became weak, to win the weak. I have become all things to all men so that by all possible means I might save some.

   These verses in no way condone living as a sinner in order to win the world. Paul would not say, "To the worldly I become worldly to win the worldly" or "To the sinner I become a sinner to win the sinners." He even mentions that he is still under Christ's law. Christ's law is to love your neighbour as you love yourself, to sell your possessions and give to the poor and to seek first the kingdom of God. The argument that ones lifestyle needs to be like that of the worldly people around them in order to interest them in the gospel is foolish. If we were to truly live sacrificial lives our love for others would reflect Christ and thus attract those ready to hear the gospel. Making it "easier" to become a Christian only leads to false conversions. Don't sell Christ short. He deserves everything from everyone.