If you are making regular contributions to an RRSP  and are receiving tax refunds every year, you are lending the government money at 0% interest! Rather than lend the government money for free, why not invest it instead?
If you are making regular personal contributions to an RRSP, have childcare expenses, make spousal (alimony) payments, or have other deductions, then too much income tax is being deducted from your paycheque.
Now you can’t just ask your employer to reduce the amount of tax withheld! Instead you need to obtain a letter of authorization from your local tax services office. To obtain this letter, you need to complete CRA form T1213 Request to Reduce Tax Deductions at Source  and send it to your local tax services office. You will need to support your request.
If you’re self-employed the situation is a little different because you may be required to pay your tax by instalments. If you are using the current calculation method and the instalment payments are too low, you may be charged interest and penalties . For more information, see the CRA Guide: P110 – Paying Your Income Tax by Instalments.