fetch top 3 bestsellers products programmatically

Mage::getResourceModel('reports/product_collection')->addOrderedQty()->addAttributeToSelect('*')->setPage(1, 3)->load();

steps to increase the performance of magento

1. Enable Gzip compression
2. Combine external CSS/JS into one file
3. Disable modules which are not used
4. MySQL Query Caching
5. Enable Magento caching
6. Disable the Magento log
7. Image Optimization

set different themes for logged in users or set different themes on runtime

 if(Mage::getSingleton('customer/session')->isLoggedIn())
{
Mage::getDesign()->setPackageName('packagename')->setTheme('themename');
}

Get cms page url in magento

<?php
$pageid=1;
echo Mage::helper('cms/page')->getPageUrl($pageid); ?>

Or
In controller or .phtml file:
<?php echo $this->getUrl('whats_new');?>

or
<?php echo Mage::helper('cms/page')->getPageUrl('url_key'); ?>

Or
In CMS Page
{{store _direct="url_key"}}

Call Newsletter form anywhere in magento

<?php 

echo $this->getLayout()->createBlock('newsletter/subscribe')->setTemplate('newsletter/subscribe.phtml')->toHtml(); 

?>

Call one .phtml file into another .phtml file in magento

<?php
echo $this->getLayout()->createBlock('core/template')->setTemplate('mobilebanners/index.phtml')->toHtml();

?>

Ratings and Reviews count in magento

<?php
$storeId = Mage::app()->getStore()->getId();
$reviewSummaryData = Mage::getModel('review/review_summary')
->setStoreId($storeId)
->load($_product->getId());
$reviewUrl=Mage::getUrl('review/product/list', array('id'=> $_product->getId()));
?>
<div class="ratings">
<div class="rating-box">
<div class="rating" style="width:<?php echo $reviewSummaryData['rating_summary'] ?>%">
</div>
<?php //echo $reviewSummaryData['reviews_count'];?>
</div>
<span class="amount"> <?php echo $this->__('%d Review(s)', $reviewSummaryData->getReviewsCount()) ?></span>
</div>

get Product URL by Id

$product_id = 85;
$_product = Mage::getModel('catalog/product')->load($product_id);
$product_url = $_product->getProductUrl();

Redirect to CMS page from phtml file

<?php echo $this->getUrl('privacy-policy'); ?>
privacy-policy is the url-key of your cms page

or
<?php echo Mage::helper('cms/page')->getPageUrl('url_key') ?>

Redirect to Category listing Page

<?php
$category = Mage::getModel('catalog/category')->load(3);
echo $category->getUrl();
?>

Magetno - Get add to cart url for any product

$_productId = '1222';
$_product = Mage::getModel('catalog/product')->load($_productId);
$_carturl = Mage::helper('checkout/cart')->getAddUrl($_product);