Below are some common points you can remember. List is endless but I do not want to make it long, so that you can remember important ones.
- Use string builder for concatenation rather than string when concatenation huge string values.
- Avoid boxing / unboxing, use generics.
- Avoid writing in line SQL queries use stored procedures.
- Choose your indexes (clustered and non-clustered) properly.
- Use Caching for data which will not change frequently.
- In ASP.NET use output cache directive for page level caching.