<input type="hidden" value="?php echo Mage::getUrl('checkout/onepage')?>" name="return_url" />

<button type="button" title="<?php echo $buttonTitle ?>"
class="button btn-cart" onclick="productBuyNowForm.submit(this)">
<span><span>Buynow</span></span></button>

var productBuyNowForm = new VarienForm('product_addtocart_form');
            productBuyNowForm.submit = function(button, url) {
                if (this.validator.validate()) {
                    var form = this.form;
                    var oldUrl = form.action;
                    alert(form.action);
                    if (url) {
                       form.action = url;
                    }

                    /* add return Url */
                     var inputReturn = document.createElement("input");
                        inputreturn.type = "hidden";
                        inputreturn.name = "return_url" ;
                        inputreturn.value = "<?php echo Mage::getUrl('checkout/onepage')?>" ;
                    document.getElementById('product_addtocart_form').appendChild(inputreturn);
                /* add return Url */
                // Append a line break
                    var e = null;
                    try {
                        this.form.submit();
                    } catch (e) {
                    }
                    this.form.action = oldUrl;
                    if (e) {
                        throw e;
                    }

                    if (button && button != 'undefined') {
                        button.disabled = true;
                    }
                }
            }.bind(productBuyNowForm);