Do you already have a reservation at a hotel? If so, make sure you let the hotel know of your special occasion in advance.
It is usually easier to get an upgrade at a chain hotel if you belong to their frequent guest program, and you often stay at those hotels identifying yourself as a program member. Some frequent guest programs give you free upgrade coupons as a part of their annual membership package--there is very often a minimum stay requirement to qualify for the upgrade.
When you get to check-in at the hotel, see how it goes. Don't act entitled in any way. Be polite. They will tell you if they are going to upgrade you. If it appears that you will not be given a free upgrade, you can still opt to pay for one. There are tactful ways to find out what can be done--I usually inquire, "Is the hotel full tonight?" If they are full, you won't be able to get an upgrade. If they are not full, you can ask to pay for an upgraded room. They will usually pull out a rates sheet to give you upgrade prices, and you can decide if a paid upgrade is what you want.