Good Risk strategy suggestion?
HistoryGamerDotCom has a 9 part YouTube tutorial on Risk strategy. To (grossly) simplify the content:


Aim for a non-isolated, not too large continent in the beginning of the game, e.g. South America
Know the chances of winning an attack. Three attackers to two defenders is my preferred ratio, as the chance of victory is high but without wasting resources
Keep an eye on people's risk cards, as this is an indication of when people are going to get a large reinforcement influx. As a general rule with many exceptions, try to delay cashing yours in, as the reinforcement count for cashing in risk cards increases over time. Note in the base game you do not gain anything extra from cashing in the risk card of a country you occupy, but I have seen computer versions that do this
Diplomacy is key! Regularly make pacts so that you won't be ganged up on. Be willing to give a little to get a little, e.g. you might want to under defend one country to keep one player on side
Use your reinforcement move to shore up your weaknesses
Don't try to expand too quickly, and don't feel you need to attack every turn. This may leave you vulnerable to a sweeping counter attack