First be sure you have installed all of your mods and plugins correctly as well as making sure they are compatible and your texture pack is supported by the mods you are using.

If you are using anything bigger than 16x16 download MCPatcher and run the program and check the HD Textures box and click patch. Then simply run minecraft like normal.

If you are still having problems:
What operating system are you on?
I'm assuming thats a laptop and you are running Windows 7.
Go to your desktop, right click "My Computer" Select Properties, Are you on 32 bit or 64 bit?
If you are on 64 bit you need to download Java 64 bit. By default most computers have 32 bit.

Your problem is if you are running Vista or Windows 7 you will not have enough RAM.
To free up more memory turn off "Superfetch"

1. Click the Start and type REGEDIT in the Start Search area.
2. Open the Registry Editor.
3. Click the plus sign next to HKEY_LOCAL_MACHINE
then SYSTEM
then CurrentControlSet
then Control
then Session Manager
then MemoryManagement
then click PrefetchParameters
4. On the right side, double-click on EnableSuperfetch
5. Give it one of the following hexadecimal values:
0 to disable Superfetch
2 to enable boot prefetching (I recommend this)
Click OK.
6. Close the Registry Editor.

Also download CCleaner to clean out temporary files that are bogging down your system. This should take about 3 minutes.

I also recommend you use Wise PC Engineer and create a System Restore Point as well as a Registry Backup, then use the Disk Cleaner, to do a broader search for useless temporary files and Registry Cleaner to get rid of problems, then the
Registry Defrag to compress your registry and give your system a small boost. This should take about 20 minutes.

Once you have done that, download Defraggler and chose Action>Advanced>Check Drive for Errors let that run, then defrag your Hard Drive to improve disk performance. This can take up to an hour or more.

If you still lag, you may need to tweak your java memory values go to your Control Panel>Programs>Java. This should open the Java Control Panel. Select the Java tab click view, and make sure the Platform is the latest version of Java and in the Runtime Parameters type in -Xmx1512m this will give java permission to use 1.5 GB (or 1512 MB) of your RAM, now click ok. Vista needs 525 MB to keep the system happy so do not exceed your RAM limits. Windows 7 needs 1 GB of RAM to maintain the system and Java limits itself to 1 GB by default so adjust according to how much memory you have.

Also you can launch your MineCraft from a .bat file. Open Notepad type in
@echo off
java -client -XX:+UseConcMarkSweepGC -XX:+DisableExplicitGC -XX:+UseAdaptiveGCBoundary -XX:MaxGCPauseMillis=500 -XX:-UseGCOverheadLimit -XX:SurvivorRatio=12 -Xnoclassgc -XX:UseSSE=3 -Xincgc -XmxINSERT AMMOUNT OF RAM YOU WANT TO BE ALLOCATED HEREM -cp Minecraft.exe net.minecraft.LauncherFrame
exit
Then save it as a .bat instead of a .txt
Simply click the bat file to launch minecraft from now on.

Also right-click your desktop and click Personalize and change your Theme to Windows Classic. If you want to go even further right click My Computer>Properties>Advanced System Settings>Performance Settings Uncheck all but Smooth Screen font and Shadow options.

You can also go to START, type in Run and then dxdiag, click ok. Go to the Display tab and notice the name of the first device. This is your graphics card model. Now go to NVidias Driver Downloads Page if it is an NVidia graphics device or go to AMD's Driver Download Page if it is a Radeon or ATI graphics card and just select the right information or auto detect the hardware and download the software and install it.

Once you are done with all of this restart your computer and you should be running like brand new again.

If none of this helps its simply time to upgrade your computer.