Reducing space between widgets in blogger.

Go to CSS Editor in Customize option

Label1{
margin-bottom: -50px !important;
}

Just replace 'Label1' with the ID of your widget. You can get the widget ID by hovering the mouse over the Edit button of that particular widget in the Blog Layout Manager. You can see its widget id name in the status bar of your browser.

#BlogArchive1{
margin-top: -50px !important;
}