SOQL Limits
1，Always use bulk syntax (see Bulk Patterns later in this chapter).
2，Use before-triggers instead of after-triggers where possible (allows
modification of fields without a SOQL query and DML update).
3，Cache query results if your design allows.
4，Include fields from related objects in a single query.
DML Limits
There is a limit to the number of DML (Database) operations you can
perform within an execution context.
Combine DML operations on each object type into a single bulk
DML operation.
If your program flow calls for DML operations at different places in
your code, don’t perform the DML operation right away. Instead,
store a reference to the object in a list, set or Map. Then perform a
DML operation on all objects at once as the last part of your
operation.
Heap Size
The size of the heap is limited during an execution context.
When performing SOQL queries, only include fields you actually
need in the query. In particular, avoid querying for long text and rich
text fields.
Instead of storing objects in a static variable, store Object IDs, then
requery when needed.
Callouts
Apex code can call out to external web services. There is a limit to the
number of callouts in an execution context.
Don’t try using callouts in a trigger context – it’s not supported.
On a Visual Force page, you can often extend your limits by placing
content in an Iframe. Each Iframe runs in its own execution context.